United trailed their rivals 3-0 at half-time, in the first leg of the Cup semi-final before Marcus Rashford pulled a goal back for the hosts.
"Not only is it not a good enough header but his reaction, to just amble back, he makes an effort to slide but it probably doesn't help de Gea.
"This isn't good enough, he has to move Mahrez out of the way there. That's fine, it's a bad header, but what is your recovery like? That's ambling back, that's jogging back and they have been punished.
"If you defend like that against Manchester City you've got no chance. The third goal, it's a great counter-attack, but there are two or three defensive mistakes from Manchester United. If you do that against Manchester City, you get punished."