He's better suited for a slower league where players of his limited strength/speed/power can thrive. I don't even think Iniesta would look good in this United team as there's no protection for a playmaker of a smaller/lighter stature. He needs at least one enforcer around him, and when that player is the slow as treacle Casemiro then he's toast.
I don't like the guy myself, find him a very whingy, cynical footballer like Robertson at Liverpool - but he's a very good footballer when in the right environment.
He's just too slow and lightweight to be doing what's expected of him. The Hollywood passes barely ever come off either, and that'd be easier for him somewhere like La Liga where he'd have more time on the ball to pick his spot.
I agree with the OP, he should be sold. He's not one of United's biggest issues, far from it, but he is the captain, on high wages and rapidly reaching the age of regression, so they should cash in on him and move for a more dynamic ATM that isn't so easily bullied out of games and doesn't look like his boots are made from tungsten whenever the pace of a game is rapid.