I think it depends on Atletico's state come summer. I personally think they've hit their ceilling as a club. I don't see them establishing themselves as an elite club anytime soon. They're always going to be in the shadow of Barca / Real and will never be more than outsiders / hipsters choice to major titles like Dortmund. This season they just don't look that great, neither does Griezmann for that matter. And with the transfer ban looming over them next season, they might see a drop after not being able to reinforce and improve in much needed positions.

Come summer, if Atletico don't look to be a position to win trophies, he has little reason to stay. United may well finish without Champions League this season but we'll be in a great position to challenge for the title and other trophies next season, more so than Atletico IMO. Similar situation to Mkhitaryan really. Dortmund weren't a position to win trophies so why stay when you could play for a much bigger club for more money who would be in a position to win trophies domestically, and then Europe in future years. Pogba, Ibra and Di Maria were signed under slightly different conditions. Pogba had big ties to United, Di Maria was being forced out of the club and we were the only team that could afford him / pay his wages. In Ibra's case, his options were China, Roma, Napoli, West Ham or us, pretty easy choice really.