Talking with ITV on their half-time show, Keane said: "It's the biggest worry for United throughout this season, they are so open, particularly in midfield. They need someone to sit, it's called game management.
"We just felt Liverpool were getting stronger but United have got to get someone to sit in there and try and protect the back four, game management, to give up the chances.
"The intensity from the attacking players, Rashford, my god, you're talking an FA Cup quarter-final, get to people, attack them, it's far too easy for Liverpool, way too simple, players taking touches, get out to him.
"Just the build-up to that [the goal], not enough aggression, what happens is a lot of teams make the mistake in that we got the numbers back but it's not good having the numbers back if you're not aggressive you have got to be aggressive in his game.
"You are gonna be punished, but United trying to play out from the back in injury time, it's criminal."