The question should be, is Ronaldo coming back as a player or a coach? cause at his age I don't think Manchester United would allow him play for the team regardless of whatever he's helped the club achieved in the past, maybe in future after Ronaldo resigns from playing then get a good coaching badge to coach Manchester United, then they could consider bringing him coming back but as a player, I don't think so. Ronaldo himself knows that coming back to Manchester United for the 3rd time won't be possible, not at this point when United is trying to build up with younger talents his only high chance of returning is as a coach and that's if he proves himself, just like Xavi Alonso has done and returned to Real Madrid as a coach.