I don't think he's ever got over his injury, physically or mentally. He would have grown up spending most of his teenage years thinking he had a glittering future; he was out of this world as a young player, so many said they'd never seen a kid as good.
Then, a bad injury at the worst time in his development, as he was just breaking through, robbed him of a vital period where he should have been learning how to adapt his skills to become an effective professional footballer. He must have been gutted and scared. All your dreams and all you've worked for out of the window.
Frustration when he came back from injury, the inability to recapture the form he had, the skills he had, probably contributed to his 'attitude problems'. He would have been desperate to get back to where he thought he should be and probably a bit put out that certain players got picked (by both Nigels) regularly when they were performing poorly.
He's a victim of circumstances - his injury, the inconsistency of managers and the failure of the club to move forwards, as well as his own attitude and it's a shame it hasn't worked out for him. Football's a tough game.