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Shaquille O'Neal lagged well behind the rest of the Boston Celtics on the short walk from the bus to the locker room on Wednesday morning.

His ailing right calf simply wouldn't allow him to keep up with the pack.

And what was painfully obvious then became official about an hour before game time: Shaq is a no-go for Boston.

O'Neal was inactive for Boston's win-or-go-home Game 5 in their Eastern Conference semifinal series against the Miami Heat on Wednesday night. Celtics coach Doc Rivers revealed earlier in the day that the 7-foot-1, 300-something-pound center's lower leg injury worsened when he tried to play in Games 3 and 4 against Miami.

"Nothing he can do," Rivers said. "It's not like he's not trying. I told our team that (Tuesday). You know, he's done everything you possibly can do to get healthy. And unfortunately for him, he just hasn't been able to do it. When he was originally injured, no one even thought it was that serious."

That was Feb. 1. O'Neal has logged 17½ minutes of court time since, and Wednesday's game was the 40th in Boston's last 43 that he missed.

"For us, it's been tough," Rivers said. "For him, it's been far tougher."

O'Neal turned 39 two months ago, cracked the 50,000-minute mark for his regular-season and playoff career last month, and has seen his numbers decline rapidly over the past two seasons.

In 75 games with Phoenix during the 2008-09 season, he scored 1,333 points in 2,254 minutes. In 89 regular-season games with Cleveland and Boston over the two following years, he's scored 977 points in 1,989 minutes.

"I know he's frustrated," Celtics forward Paul Pierce said. "The man's been through a lot of wars, a lot of battles."

There is speculation that those days may be nearing an end — especially if Boston's season ends against Miami. O'Neal is under contract at $1.4 million for next year with Boston, though it's still unclear if he would want to endure another injury-filled campaign.

Rivers does not sound like he knows if this season will be the end of the line for O'Neal.

"I think it's too early to even talk about it right now," Rivers said. "I've learned personally that you never try to make any decision during the heat of the battle. Emotionally, you're always going to make the wrong choice then. I'll think he'll walk away for the summer and decide what he wants to do. I just know this has been emotionally draining to him. ... He feels awful about this."

There would be some irony if O'Neal's career ends against Miami. The Heat were his first NBA opponent when he entered the league in 1992, plus presented him with his fourth championship ring in 2006 and honored him by prominently splashing his picture all over the hallways around the Miami locker room — part of what the team calls "Championship Alley."

Ironic, too, that O'Neal often referred to current Miami stars Dwyane Wade and LeBron James as his pupils. O'Neal spent the 2009-10 season with James in Cleveland.

Wade and James both paid tribute to O'Neal on Wednesday, when asked about his legacy.

"Shaq is one of the most dominant players in our time," Wade said. "Who knows how long he's going to play, but you can never take away anything he's done in this game as a champion. ... He's a legend. He's a living legend."

Added James: "He made fans believe that they were at one with him."