While it's hard to see past the historic turnaround and 17th NBA title, it was only a year ago that many basketball pundits were questioning the depth of the Boston Celtics bench. There was no way of knowing that veteran journeymen like James Posey, Eddie House and P.J. Brown would come up with clutch shots during the playoffs or that a young talent like Leon Powe would emerge in the Finals.

But winning a championship doesn't keep the Celtics immune from skepticism and doubt. Posey and Brown have since left Boston, leaving a huge gap on the bench. Now even head coach Doc Rivers questions, what is the identity of the second unit? Only those involved have the answer.

“We're getting the job done and expect nothing less,” said Powe. “We're going out there defending hard and we're going to keep the lead or increase the lead. That's our goal as a second unit.”

Last season the Celtics starting five demonstrated an unwavering commitment to success that trickled down to the last man off the bench. This infectious drive to win was made possible by a team-first mentality (“Ubuntu”), which the reserves are embracing again.

“I think we've got a bunch of guys who are coming in trying to contribute, instead of just one or two, maybe three at the most, and then you don't know what you're going to get from the other guys,” said House. “I think everybody knows their role and they're just coming in playing their role to the best of their ability.”

Tony Allen agrees with House. He has significantly improved his offensive skills over the summer to give the Celtics more weapons off the bench.

“I'm real excited,” said Allen. “There are some more things I want to work on my game going into the regular season ... I'm just trying to be a piece of the puzzle.”

Rivers has not announced a definitive second unit and the combination will change from game to game depending on the competition. Heading into Opening Night, though, signs point to Allen, House, Powe, Glen Davis and Gabe Pruitt. Each of them played for the Celtics last season. Each of them knows what it takes to win a ring.

“We're still going to come in there and bring the same intensity, work hard, try to fight for every loose ball, and try to get defensive stops,” said Powe. “Our offense, it'll come. But we mostly focus on the defensive end like we did last year and that's what helped us out a whole lot going through our run.”

For sophomore Pruitt, this is his first opportunity to be part of the second unit. He spent most of his rookie season in the NBA Development League and took notes from the bench when he was called back to Boston. In the last month he proved he has embraced the hustle mentality of the Celtics reserves.

“Everybody's coming in with the mindset of team first,” Pruitt said. “Nobody on the second unit really thinks me first. But it's easy when you've got scorers like Eddie and T.A. because we know who we have to get open to get us points, but they know they have to come down on their part of the team defense concept.”

The second unit has no choice but to pride itself on defense. Every practice it is their job to stop three of the league's most able scorers in Paul Pierce, Ray Allen and Kevin Garnett. These veterans offer no leniency for younger players and accept nothing but maximum effort.

“Basically Kevin, Paul, Ray help us tremendously with the way they play and the motivation that they have,” Davis said. “We've got a lot of young guys in our second group so we're just learning each day.”

One of the biggest keys to last season's success was something that cannot be learned, though. It was Posey's basketball instinct that resulted in dramatic game-changing plays and countless possessions that narrowly escaped being lost out of bounds. Accounting for that intensity is perhaps the second unit's biggest challenge.

“It does feel weird without Posey,” Powe said. “He was the heart of the second unit and he held everything together. If something came out of line, he was there to put it right back into place. All of us as a whole know we’ve got to take on that role. We can't let one another slack and don't say nothing. We've got to hold each other accountable.”

Just as Posey did, this season's second unit plans to attack each game as if they had been on the court since tip-off. The starters expect a seamless transition between substitutions and Rivers looks for his reserves to play with the same intensity as the first unit that tests them in practice.

“We come out into the game really aggressive from the start,” said Pruitt. “We don't work our way into the game.”

As the Celtics begin defending their NBA title this week, this group of players will set out to uphold their label as the most dominating second unit in the league. And the best way to do so on this team-first squad is simply by identifying themselves as Celtics.

Jessica Camerato covers the Celtics for WEEI.com.