If Pierce were healthy I'd say the C's had a 40% of beating Miami.
With Pierce unable to get lift on his jump shots, the C's chances of winning drop to 10% or so.
The C's could still do it if KG maintains his current level of play, Rondo picks his up a little - mostly in the dominating end to end speed way. He's already playing at a high level though. And then thirdly the 3 point shooters need to be on. Any 2 of Pierce, Allen, Bradley, Pietrus, Dooling, Pavlovic (if required to play) need to hit a couple 3's per game.
With Bosh potentially hobbled, the C's chances increase significantly. He's the only one that can match up with KG. Haslem is limited and OK, but the rest of the Heat bigs are going to get torched by KG if forced to play him.