How many players do United have that are world class and in their prime?
De Gea - nailed on world class player, who's also almost certainly leaving for Madrid.
Absolutely no one from their defence worth talking about.
Schweinsteiger - certainly not in his prime, but still a good player, serious question marks over whether he's still a top player or not though given his performances last season, his age and injury record.
Di Maria - top player in his prime, but off the back of a very poor season and also perhaps leaving for Bayern or PSG.
Rooney - not sure I'd call him one of the best players in his position in the game right now, would you? That said, I still fancy him to score a fair number of goals if he's played up front consistently next season.
So one, with two or three questionable ones for different reasons. Compare that to Chelsea; Courtois, Terry (old but not past it, not last season anyway), Ivanovich, Matic, Fabregas, Hazard, Costa all in their primes and the among the best players in their position - and United, despite some good signings, are still a million miles off.
They need players that are near the top of their position and in their prime - but they can't get them to come. Maybe they'll get them in the remainder of the window but I very much doubt it. More likely more of the same - ie. Otamendi not Ramos, Benteke not Benzema etc.
And the funny thing is, the two players on that list that are in their primes; De Gea and Di Maria, they're struggling badly to keep hold of. That's because United, for various different reasons, is not the place to go and play if you're a top player right now. There are bigger and better options out there.