I don't begrudge Ronaldo wanting to leave us. Realistically, he hasn't got much longer at the top so if he thinks he can get a move to a club that will offer a chance to compete for domestic and possibly continental in the near future then it makes sense for him to take it.
I just rue the fact that we ever decided to bring him back in the first place. A load of people talk about how we'd have been relegated without him but on the few occasions we got to see United with him not in the side (e.g. first-half against Everton at home), I don't think we were any worse off. We've essentially spent about 35 million quid (minus any money we might get for him if we do decide to sell) for one year of a guy who, whilst he's scored a lot, has not actually improved the side at all (in fact, it got considerably worse when he joined), has reportedly contributed to destabilising the dressing room, and has now asked to leave after one season.
In my eyes, if he wants to go we shouldn't stand in his way. In the long-run, I think it'll be a huge benefit to us.