This Season, Celtics Are at Once Hobbling and Walking Tall WALTHAM, Mass. - The Celtics played the first game of the lockout-shortened 2011-12 season, opening against the Knicks at Madison Square Garden on Dec. 25. But the team's coach, Doc Rivers, says its real season began two months later, after the All-Star break, with the Celtics" record an unimpressive 15-17 and the team on a second five-game losing streak. Rivers revamped his starting lineup and tightened his rotation. The results were immediate and striking. The Celtics went 24-10 and became only the fourth team in N.B.A. history to win its division after having a losing record at the All-Star break, according to the Elias Sports Bureau. The other three, the 1975-76 Milwaukee Bucks, the 1971-72 Baltimore Bullets and the 1956-57 St. Louis Hawks, all finished the season with losing records. The Celtics finished 39-27 to win the Atlantic Division by three games, earning the No. 4 seed in the Eastern Conference. They will open the playoffs Sunday night in Atlanta. "After the All-Star break, that's when our season really turned into a season," Rivers said. That's how I feel. And, obviously, by the way we played. The Celtics team that enters the playoffs is vastly different from the one the Knicks saw in the season opener. Paul Pierce missed that game with a heel problem. He returned five days later and ended up leading the team in scoring. The projected starting center and backup center at the start of the season, Jermaine O'Neal and Chris Wilcox, both sustained season-ending injuries and have been waived. Their absence prompted Rivers to move Kevin Garnett to center, a position he has played in 36 games this season. The Celtics were 25-11 in those. Garnett averaged 2.6 more points per game and 1.1 more rebounds per game as a center. Garnett will go down in history as one of the league's greatest power forwards, but for now he is a center. After the Celtics" regular-season finale against Milwaukee on Thursday, he declared that he hated the position. Then he smiled. "It's not a preference of mine," he said. But it's something my team needs, so I don't think about it. Rivers closely monitors Garnett's minutes and regularly pulls him midway through the first quarter, regardless of how well he may be playing. The player who replaces Garnett, the rookie center Greg Stiemsma, did not get off the bench in the season opener but is now a rotation regular. Stiemsma, who toiled overseas and in the N.B.A."s Development League before making his first N.B.A. roster at the age of 26, has proved to be a valuable pickup. He finished the season ranked second in the N.B.A. to Oklahoma City's Serge Ibaka in blocked shots per 48 minutes. Rivers also has the veteran swingman Mickael Pietrus at his disposal. Pietrus signed with the Celtics the day before the season opener and was not available for it. He has bounced back from a concussion in late March and gives the Celtics a reserve who plays tough defense and can hit 3-pointers. But the biggest difference in the Celtics since the All-Star break has been the play of the second-year guard Avery Bradley. He has been a revelation, taking over the starting position from the injured Ray Allen. Defense had always been Bradley's undeniable strength; Miami Coach Erik Spoelstra called him the best on-the-ball defender in the league. But in addition to his stifling man-to-man coverage, he has blossomed as an offensive player, making 3-pointers and moving without the ball to get easy layups from Rajon Rondo, the N.B.A. assists leader. During the last 19 games, Bradley averaged 15.1 points and shot 52.3 percent from the field, and 48.8 percent on 3-point shots. He averaged 5 minutes 32 seconds a game in December and 32:55 a game in April. "I always go back to those two guys," Rivers said, speaking of Pietrus and Bradley. In my opinion, they changed our team. They allowed us to do things this year we didn't think we'd be able to do, both offensively and defensively. They have been huge for us. Still, even up to the March 15 trading deadline, there was sufficient concern to prompt General Manager Danny Ainge to consider dismantling the core. He nearly traded Allen to Memphis, and a Pierce-to-the-Nets deal was explored. But Ainge decided to keep the unit together for what may be its last run. In turning around their season, the Celtics focused on their top priority since their last title team, in 2007-8: defense. After a shaky start that could be attributed to a dearth of practices, to injuries and to the integration of seven new players, the team finally started to jell defensively after the All-Star Game. The Celtics finished No. 1 in the league in defensive field-goal percentage as well as defensive 3-point field-goal percentage. They were second to Chicago in points allowed. This helped offset the handicap of being one of the league's worst rebounding teams. The one concern heading into the Atlanta series was the status of Allen, who Rivers said was not expected to play Game 1. Allen missed the last nine games with a sore right ankle and has been using a walking boot the last two weeks.