Sure, in the Finals every game is something of a must win. But I don't think there's anything that makes this game more important for the Celtics than the Lakers. If the Celts were weak-minded and ran the risk of having their confidence roll downhill like a snowball getting bigger and bigger then yeah, I'd be worried. But this team should have put all doubts about weak-mindedness to rest after the Atlanta, Cleveland, and Detroit series. They know they're the better team. I don't think it'll come to this, but I think the Celts are strong enough mentally to come back even if the Lakers were to take a 3-2 series lead. But again, I don't think that will happen.
On the other hand, this is the exact reason why game 3 and game 4 are so important for the Lakers. I think they can be a really good team when they have confidence, but if you put them into a 3-1 hole, I think they'll go belly up and this thing will be over in 5. That's why I'm a big advocate of kicking them while they're down.