Honestly, a lot of incomprehension in this thread.
He was a limited (with potential) player in his first years in Atletico and then grew and grew more especially last season and this one.
I can't help but think people saw a ghanian player playing in midfield, first words are "limited", "no passing ability" or "destroyer" whereas he's anything of those. He's a pure 8, with a good engine, a good tactical awarness (he really stepped up in this area I think), a frightening strike in him and a nice eye for the between the lines passes. I think I see him as a less "blood & thunder" Matuidi but with way better pure technical abilities or maybe what Fred would be if Fred was actually a very good player performing.
Honestly go to an Atleti forum or on Atletico reddit or even Atletico instagram (or his) and you'll see how much he's valuated by Atletico fans, who wanted him to start in midfield (not Koke, not Saul) with Rodri since the start of the season. I think they will cause uproar if he leaves as he's seen as a "cantenaro" like Saul and Koke.
But I think it will be very hard for United to get him, 50 millions is a great price that a lot of teams should look into for a 26 years old midfielder entering his prime, but I have a feeling that if Simeone promise to make him a leader for a new look Atletico (and a new contract), he'll stay.