Yes Basti's legs are gone, yes he is nowhere near his peak form of 12/13, yes he is injury-prone and slow these days, but (1) IF he can manage to stay healthy and (2) IF he accepts the bench role and (3) IF he is willing to accept a paycut I do see some value in him in terms of helping stablize our midfield and filling the void of the CM role a bit. But I don't see it happening.
He went to Manchester United because growing up he and his older brother Tobias were ManU fans, the latest I read was he is still fighting to stay at Manchester United and if that fails, I see him going to the MLS for a few years and finally returning to Bayern in some other capacity.