Someone else on TEF mentioned that the cut on his foot -- the nasty one he got during the Sunderland match -- still hasn't healed.
Something like that could certainly play a part in why he's looked so uncomfortable defending in space. Could also play a part in having a tough time mentally as well.
We may have paid too much but I have time for Keane. He's young enough to sort out whatever problems he's having currently. (Plus, I really think he looks useful on the ball. Would be nice to have a big CB with the ability to play out from the back some. Has to sort the positioning and 1 v. 1 defending first, though.)