There are three possible options:
1) He has a very severe injury
2) They rested him for the finals (haha)
3) He's a pea-heart
It can't be anything else. If it was a minor niggle he would have played given the stakes, so that isn't a possibility.
I would be astonished if they rested him in truth. He probably had a legit back strain that restricted him enough that playing him wouldn't be feasible. Even Carlton wouldn't be that incompetent.