He hasn't exactly covered himself in glory so far.
Thing is, more than one thing can be true at the same time (radical concept, I know).
He can be suffering from a system that - clearly - isn't working optimally at the moment and, at the same time, be making unnecessary mistakes on a purely individual level.
The latter is something a player with a supposedly strong mentality can't be easily excused for. If he can't keep his head, if the pressure is getting to him - say - that's kind of a deal breaker in terms of the kind of player many expected to get for that sort of money.
The odd clanger, not being a brilliant shot stopper - and so forth: that was expected. There was always a tradeoff involved regarding the inevitable comparisons with De Gea, and reasonable fans were prepared for this.
So far, though, he's simply looked a bit shit for a big signing (which he is *). Again, this is undoubtedly because of both systemic and inividual factors. But the latter are on him - how could they not be?
* He's the third most expensive keeper ever in pure numbers, and even if you adjust those numbers for inflation and whatnot, he's got to be pretty high up the list.