Plan B, the one with Jermaine O'Neal at starting center, went into effect yesterday. Shaquille O'Neal limped off the floor only five minutes into his morning test run, and his namesake's return from left knee surgery became the day's only relevant comeback story. Paint defense has become a major Celtics concern, and a healthy Jermaine O'Neal just might be the player best equipped to address the problem. Some of Paul Pierce's first playoff experiences came against O'Neal's Pacers. The Celtics captain knows what O'Neal, when healthy, can provide. "He definitely can be a guy who can block shots and guard his man one-on-one in the post," Pierce said. "He does so many things when he's right. When he's on, he's one of the best defensive centers, if not the best, in the league.