Do you really think that if we signed him, and he wasn't good enough, we'd be playing him week in week out?
If we sign him, and he's not good enough, at the very least we have a very influential figure who cares deeply about the club and can help guide those around him and perhaps inspire some passion from the other players. We don't have to play him. But if we do sign him and he's back to pre-illness level? We have a potential captain and Premier League experienced CM to lead us in a turbulent period and perhaps put in some performances that will help us get back into the Premier League.
It's a fairly win-win scenario, either get a Premier League level player in the Championship, or an influential figure that the club has been sorely lacking ever since he left in the first place, that doesn't have to play if he's not good enough.