COULD be one of Owen, Barton, Martins or one of Cacapa or Rozehnal.
If Owen gets back to fitness and starts scoring for fun, he'd be a key-player.
If Barton lives up to my expectations and is the box-to-box driving force in midfield, he could be a key-player.
If Martins finds his role and learns how to use his strengths, he could, eventually, become a key-player
If/when Cacapa/Roz learns the English game, and learns to control and command our defence, one of them could be a key player.
It's just too early to say, IMO, especially with so many new faces. Could well be Geremi, Milner, Zoggy or Enrique as well, though I think all of them are less likely than the once I just described