You won't be allowed to leave at the same time. Last ones in before the game should be the last ones out, and first ones in the first ones out, also per stand and per section. Maybe they might contemplate keeping the drink&food stands closed, and that your'e also not allowed to leave your seat during HT. Take a piss at home before the game, and if you desperately need to go during the game, you need to exit the stadium and can't re-enter. Harsh, but the only workable solution - you simply can't have fans arbitrarily choosing when to go for a piss during the game because in that case you can't control or keep your social distancing rules.
I dunno, maybe it's just too complicated. But if there's even a slight chance that this would work, I'd imagine clubs are already looking into making it happen. Even at 1/3 stadium capacity, the gate revenue of Old Trafford amounts to a million or so, probably.