Michael will be fine. Michael's energy level's on stage have always been off the charts. One thing you'll notice when watching past performances of his is that he never actually stops moving. Even when he isn't dancing in structure or performing an actual step, he's strutting across the stage. Not just walking, but moving with intensity. Whether he'll do that now is anyone's guess. Maybe he'll actually take rest periods where he'll simply sing a song while standing still. Maybe. But I also think Michael has a hard time resisting some kind of movement. I mean, this is the same guy who will bust out a move or two while performing "Heal the World".
The other thing people should realize about Michael's dancing though is that it always has been quality over quantity and subtle steps which comprised his style. He isn't like some of these idiots today who try to execute as many steps as they can during one performances and make them big and dumb, like doing back flips or spinning around on their head. Michael's genius as a dancer is that he's able to take simple, minuet steps and make them look more spectacular then anything an athletic dancer can produce. The way Michael will move any part of his body just a fraction of a centermeter, but do it with so much precisian and power that it looks like the earth just shifted, that's part of what seperates him from being just a good dancer and being a great dancer.