Couldn't see him leaving unless it was for champions league football or Everton. He's regarded by most as a player that should be starting in the World Cup so there's no motivation in that department.
I wouldn't begrudge him a move, he's stuck with us through some **** times, but I don't see united as a wise move for him. Only if they avoid the Europa league and can focus solely on the league like Liverpool this season, but even then, they're an ageing side who are probably a few years of hard work away from mounting a serious title challenge again.