Everybody wants someone to blame for this 76ers season that has fallen off the tracks. And really, there’s no need to look any farther than the 7-foot mass of twisted and tangled knee ligaments whose season so far consists of two bum knees, one practice and no games. That’s the easy answer. Andrew Bynum’s inability to play this season has certainly hamstrung the Sixers, who planned their season around a 7-foot, 285-pound hulking presence in the middle. But that doesn’t explain everything, including how the Sixers have lost nine of their last 10 to pretty much make the rest of this season irrelevant. After all, the Sixers aren’t the only team to lose their star player this season. The Boston Celtics lost Rajon Rondo on Jan. 27 for the rest of the season after he tore ligaments in his knee. The Celtics are 12-4 since then after beating the Sixers 109-101 on Tuesday. The difference? Leadership. And that is something the Sixers have to address whether Bynum ever plays a game for the Sixers or not. Even when he’s been around, Bynum hasn’t displayed that quality, whether he’s showing up late for a game or leaving practice as soon as it’s over. “The core of that team, they’ve been to the pinnacle of basketball,” Sixers guard Royal Ivey said about the Celtics. “They know how to win tough games. They’ve played at a whole different level. When you’ve got young guys like we do, that’s something you have to learn, and you can only do that with experience.”