It would depend on the kind of fracture and which bone it's in. Some players have handled it, others have't been able to play.
The fact that Koivu is using crutches, a foot brace and is not even going to attempt to play for the rest of the regular season assures me that he is going everything to get his foot to heal as fast as possible. It's not his first injury. He has learned what to do to heal as fast as possible.
So if the swelling is down and there's no chance to worsen the injury, I guaranty it's going to be cortisone shots till the end of the play-offs..
I also think that there may be a rule of thumb..
The Habs may try the first game without Saku..
If they win, the second without Saku. If they win, the third.. etc.. till Saku feels good enough to play without too many pain killers..
But if they loose the first game, Saku plays the second if he can skate with lots of pain killers..
I think I remember Pronger playing in the playoffs with extra "shielding" one one of his skates. I am confident that Saku will play in the playoffs. Maybe he wont play in the first couple of games or the first round even depending on how things go but if he is needed he will find a way to play.