The decision that defined the Celtics’ 2011-12 season wasn’t the easiest sell. It was something coach Doc Rivers had thought about when training camp opened, but it didn’t become a reality until the All-Star break when he moved Kevin Garnett to center and Brandon Bass into the starting lineup.

“Our guys were not real convinced about that move,” Rivers said. “Kevin was more worried about our defense, our anchor, and all that stuff. I don’t think anyone was real sure about that. I would say they were 50 percent at best. It wasn’t Kevin to [center], it was taking a big off the floor and going smaller. They were all concerned about it. They questioned me quite a bit on that decision.”

Nothing alleviates concerns like winning, and after the Celtics won five straight games following the break with Garnett seamlessly making the transition to center, this major transformation became accepted quickly. Lost in all this was Bass, the powerful forward with a guard’s touch on his signature rainbow jumper. 

Since his arrival via the Glen Davis trade -- one of the better moves Danny Ainge has made in recent years -- Bass has been notable for his workmanlike consistency. He rarely has bad days, and he’s not one to step outside his role. But as easy as he’s made it look at times, this has still been a transition and not always a smooth one.

“Every day is a learning experience for me,” Bass said. “I feel like I’ve got so much room to grow, I’m not even paying attention to getting comfortable with any type of situation. So, that’s me.”

Bass was an outsider, thrust into a prominent role with four veteran All-Stars. Over the last few years, the fifth player in the starting lineup has changed from Kendrick Perkins to Shaquille O’Neal, Nenad Krstic, Jermaine O’Neal and Davis. Bass not only had to get used to a new system, but also his strong-willed teammates and their sharp tongues.

“Everybody was on me about this, about that,” Bass said back in March. “I’m finally getting it. I knew it was going to take some time. I guess all the guys knew it was going to take some time as well.”

Bass soon learned that he didn’t just have to take it, that he could dish it out as well. In their masochistic way, the veteran Celtics kind of like it. In other words, he could pull a James Posey, who let it be known early in his brief tenure that he wouldn’t be intimidated.

“Posey was so great because he did it on the first day of camp,” Rivers said. “He didn’t wait. They don’t mind it. They don’t care. They want to be communicated to. It shouldn’t be a one-way street. It’s a team. All the new guys struggle with that.”

And now?

“He speaks his mind,” Rivers said. “On the court he speaks his mind. What I think his biggest improvement is he allows himself to listen now. Before he was so frustrated because he wanted to say something. So, he could never hear what they were saying because he was looking for the answer. Now he listens to the question and he gives an answer.”

As Bass began to learn the culture, he also began to figure things out on the court. Over a stretch of 20 games from March 19 to April 22, he averaged almost 15 points and seven rebounds per game. He recorded 17 double-digit scoring games and five double-doubles, while the Celtics rolled to a 14-6 record and took command of the Atlantic Division.

Among big men, Bass takes more jumpers from 16 to 23 feet than all but five players, and at 48 percent, his shooting percentage is second only to Dirk Nowitzki. He’s also become a better finisher at the rim, where he’s made 63 percent of his chances the last two months, up from 48 earlier in the season.

With Bass operating mainly in pick-and-pops, he helps maintain their precious spacing that is so valuable to making the offense function. The Celtics knew all that when they made the switch, but it’s been his efforts on defense that have been so critical. Long known as a tough one-on-one defender who sometimes gets lost in team schemes, Bass has improved greatly since playing alongside Garnett.

“Our defense is quicker,” Rivers said. “We’re not as big so we give up things there, but Kevin and Brandon are so interchangeable that in some ways our defense has gotten much better. The stats say we’ve gotten way better.”

Much of that credit for the defense kicking into overdrive has gone to Avery Bradley -- and deservedly so -- but Bass has his own role to play in their system that has adapted from its core principles to allow for more versatility. Bass is strong enough to guard some centers, which allows Garnett to guard high-scoring fours. Rivers also likes that Bass can switch at the end of the shot clock, which is something their centers haven’t been able to do in recent years.

Take their last meeting with the Hawks at the Garden -- who the Celtics will see in the first round of the playoffs. Bass guarded 7-footer Zaza Pachulia for most of night, which allowed Garnett to check Josh Smith. Pachulia is injured and likely to miss at least part of the first round, but the Celtics will likely maneuver Garnett on to Smith as much as they can, leaving Bass to do some of the heavy work inside.

As they enter the playoffs, there are still questions about roles mainly due to health. Will Bradley be able to carry his impressive second half into the hot glare of the postseason? Will Ray Allen and Mickael Pietrus be healthy enough to contribute? But there are no questions about the frontcourt where Brandon Bass has quietly answered their concerns and made himself into a valuable starter.