WALTHAM — Much has been made about the empty banner that hangs in the corner of the Celtics practice facility. It is a physical manifestation of the expectations that always hover around this franchise, and it is also a not-so-subtle reminder that filling in the unwritten words is the goal for this year.

Not next year, or the season after that. This season. 2009-10.  

Far less has been made about the banner that sits immediately to the left. The one from the 2008 season.

Unlike the other 16 that line the practice court, the 2008 banner has yet to show the effects of age. Its edges are crisper and cleaner. It has not yet begun to bunch and billow like the ones that commemorate the other great eras in franchise history — from Russell and Cousy to Havlicek and Cowens to Bird and McHale — that have long since passed into legend.

In many ways, the story of the 2008 banner still is being told because the core group responsible for it is still hard at work on the court down below, trying to do it all over again and add another chapter to its legacy.

When Kevin Garnett and Ray Allen joined Paul Pierce and the Celtics, the talk was of a three-year window to compete for a championship. Being the pragmatic person that he is, Doc Rivers never wanted to hear about windows or steps. He wanted to win now and it remains an underappreciated aspect of the psychological job Rivers did as a coach during that 2007-08 season that he was able to convince his players that this indeed was the time.

“Our goal is to win it,” Rivers said. “We make no bones about that. We don’t look for next year or the year after. If you have a chance to do something special, you better keep your focus on the here and the now.”

Rivers said that on Sunday, just two days before the Celtics open the season against Cleveland. He could have said it exactly the same way two years ago and it would have been just as true. The Celtics’ window to win is the present tense, just as it was in 2007-08.

But unlike that first season of the modern-day Big Three, the future is cloudier and more uncertain.

In relative terms, Garnett, Pierce and Allen aren’t getting any younger. Injuries complicated things last year, as they tend to do with older players. In concrete terms, Allen is entering the last year of his contract, while Pierce has an opt-out option in his. Garnett is safely signed and secure, but he is coming back from the most serious injury of his career.

As the final year of that three-year window begins, it is only fair to ask: Is this the last season for the Big Three?

MAKING SENSE OF UNCERTAINTY

While it certainly doesn’t end there, the final answer to that question ultimately starts with Ray Allen. Pierce’s contract situation will resolve itself in due time, and while he has publicly acknowledged the reality that he might be open to changing the structure of his deal, much has to happen before even beginning those kinds of conversations.

Setting aside Pierce’s situation, the Celtics will finish the season with four players under contract. Garnett, Rasheed Wallace, Glen Davis and Kendrick Perkins are all signed through at least 2011.

That’s when things get hazy. Pierce has his option. Rajon Rondo will either be extended or become a restricted free agent. The Celtics will also have the ability to control the fates of Bill Walker, J.R. Giddens and Lester Hudson. Ray Allen, Eddie House, Marquis Daniels, Brian Scalabrine, Shelden Williams and Tony Allen all will be unrestricted free agents.

Those are all things beyond Ray Allen’s control, really beyond anyone’s control besides Danny Ainge, the rest of the Celtics front office and various agents. So, Allen doesn’t worry about it. It’s an incomplete answer to a fairly straightforward question, but Allen knows that if the Celtics stay healthy and if they’re able to fill in the blanks on the empty banner, the rest will probably take care of itself.

“When I signed my contract it was for five years,” Allen said the other day. “I don’t worry about it.”

And that’s about as expansive as he’s going to get on it.

“He’s gone through all that,” Rivers said. “You’re not great unless you’re very comfortable in your own skin.”

Make no mistake, Allen is very comfortable with himself, which is different than saying that he’s comfortable as a player. He is a constant tinkerer obsessed with the simple art of shooting a basketball, an art that has paid him very well over the years, and an art that any sane person would say that he mastered long ago.

But there is art and there is the artist, and for the artist the perfect form is a never-ending quest, which is why Allen spent part of the summer reading about snipers.

“If their heart beats too much, they’ll get excited and miss their target,” Allen said. “I added that into thinking about my free throw shooting. Slow your heart down so you don’t get too excited and miss a free throw. There’s a lot of circumstances that you can allow in that force you to miss. It’s routine. Everything is routine.”

That last sentence encapsulates the essence of Allen on the basketball court. He works constantly at his routine, which then makes what is essentially a 50-50 proposition less a matter of luck, and more one of circumstance. He shoots the basketball and it either goes in or it doesn’t, but it all goes back to routine.

It’s why Allen himself never talks about “slumps” even when everyone else does. A slump is based on luck, and Allen prepares to keep luck from entering into the equation. Luck is something that is beyond his control and doesn’t really interest him all that much. So, what does?

“I’ve been asked several times over the years what is my challenge,” Allen said. “Playing 82 games is a challenge. That’s my goal every year. To play 82 games. As easy as that sounds, you’re talking about a lot of games, travel, staying healthy. Playing 82 games is an accomplishment. With what we went through last year, you’re talking about health. We had a lot of injuries last year. That’s a challenge.”

It’s ridiculous, right? For a player of Allen’s stature who is entering the final year of his contract while playing on a team that can win a championship, a concern first and foremost about the number of games he plays doesn’t appear to make any sense. Unless you consider that to think about anything else makes about as much sense as trying to predict the future.

“A lot of players don’t know how to stay focused,” Allen said. “You have the present moment and deal with that and that other stuff takes care of itself.

“This, for us, is the prize right here. So many people come up and say, ‘Are you guys going to win it all this year?’ Let’s talk about the regular season. You want to fast forward through all the bad times and get to the good times. Well, if you do that then you never learn anything.”

It’s not like Allen is on his own metaphorical island here either. All the Celtics veterans think like that. The last few days, people have been trying to come up with various ways to ask them to compare this year’s team with the 2007-08 version, and all those attempts have been met with some variation of a stone wall. They just won’t do it. 

Except for one small thing. 

“This is a really focused team,” Rivers said. “They understand what they’re here for. They stand out in that regard. That team two years [ago] had that same feel. There wasn’t a lot of other stuff going on. You get that same sense of urgency from this basketball team.”

CONVENTIONAL WISDOM

No one is ever really sure where conventional wisdom starts and reality begins, but when they do intersect, it generally leads to a host of accepted norms. For example, the conventional wisdom in the NBA is that it takes years for a team to be comfortable playing together before it can seriously challenge for a championship.

The proof is right there in the Book of Stern where it states that before they could be great, the Pistons had to learn how to beat the Celtics and the Bulls had to learn how to beat the Pistons and so on.

Of course this ignores the experiences of the 2008 Celtics, who turned conventional wisdom on its head and took great delight in doing so. That Celtics team was not supposed to win a championship, at least not that fast. It was supposed to be good, maybe good enough to reach the Eastern Conference finals or maybe even good enough to advance to the NBA Finals. We forget all that because of what that team accomplished, but right up until the last, people had their doubts.

The doubt drives people like Pierce, Garnett and Allen because they are hyper-competitive by nature, and the re-telling makes for a good story. “No one expected us to be here,” and all that. But the critics and the doubters, the purveyors of conventional wisdom, were not the reason the three bonded so quickly.

What Garnett, Allen and Pierce quickly realized was that they were kindred spirits in a very small and insular world. They recognized right from the beginning that the other two complemented them and their talents and in essence, made their lives easier.

“Paul makes things easier,” Garnett said earlier in camp. “I’m not going to lie about that. He not only makes things easier, but simpler. Offensively things get more simplified, plus IQ-wise, a lot of things he and I don’t need to talk. We have a bond and a chemistry that’s unspoken.”

“I have history with Paul and I have history with Ray,” Garnett continued. “All of us knew each other. We just hadn’t been on the same teams. Paul and I were the same high school class, playing against each other, played with each other. Ray and I grew up in South Carolina. So I know those guys. I know their families. To get in the league and have those guys as teammates on this level is a more personal thing for us.”

The trio’s shared experiences made the whole transition easier, but what they also had from the outset was trust in each other on the basketball court. Sure, there were little things that had to be carved out, and still have to be tended to and nurtured over time, but their trust in the big picture was and is absolute.  

“They’re focused on playing,” Rivers said. “That takes years. You don’t see a lot of young teams winning championships. There’s a reason for that. They’re still learning how to play. By nature, young players don’t have a sense of urgency. They think, ‘Oh, next year will be good.’ The older guys understand this year is precious.”

That is the Celtics’ biggest advantage, and also their biggest weakness. That kind of understanding only comes with age and wisdom, but that kind of experience also only comes with time, and time is not on their side.

The conventional wisdom for this season’s Celtics is that they are very good and (again) if they stay healthy, they will have a chance to compete for a championship. The problem is that not many people actually expect that they will stay upright and in one piece for the duration of the season. Not surprisingly, that sentiment is not one that’s shared among the Celtics, and that’s when the competitor comes out.

“I don’t know what they think,” Allen said. “We as a group, collectively, we take care of ourselves. Everyone knows what their job is. Tell me that I’m getting too old. Give me the ball and let’s see what happens.”

THE END?

It will end at some point. We all know that. For Allen, the end of his time with the Celtics may come sooner than the others, and he knows that, too. That is beyond his, or their, control.

But if this era is to continue, then they have to win. It’s really that simple. Presumably, each of them might have to make sacrifices, either financially or in terms of shots or minutes, but if they don’t win, then it won’t make much sense to keep them together, especially if they don’t win because they can’t stay healthy anymore.

As we enter the third year of the Big Three era, it’s worth remembering that health is the only thing that has really defeated them. In the two years they have been together, Paul Pierce, Ray Allen and Kevin Garnett have been part of the best teams in the NBA. They won a championship together and they started last season winning 27 of their first 29 games when everyone associated with the team acknowledged that they weren’t playing all that well.

Then Garnett got hurt, Allen had a hamstring issue and Pierce appeared to be worn down — and they still almost beat Orlando, which went on to win the Eastern Conference championship. All that was nice in some ways but drastically unfulfilling in others.

That’s all past now. With that unadorned banner staring down at them, Pierce, Garnett and Allen know that if they can stay healthy they can compete for a championship, and if they can compete for a championship they can continued to add to their legacy.

It’s the only way this era can end happily, and it’s also the only way it can continue.