The Red Devils were utterly hopeless throughout the game, with Mohamed Salah grabbing a hat-trick for the visiting side.
Paul Pogba was also shown a red card in the second half after coming on as a substitute after the interval.
Yeah, it wasn't a great day for United...
All the positivity from the team's magnificent 3-2 comeback victory versus Atalanta has evaporated and it's hard to see where Ole Gunnar Solskjaer goes from here.
The manager didn't exactly help himself versus a brilliant Liverpool side by playing the same XI that started on Wednesday in European action.
They may have eventually won the match, but the first half display that evening was woeful.
United legend Paul Scholes raised that point in the BT Sport studio after the full-time whistle had been blown.
The player-turned-pundit made his concerns public knowledge and predicted that his beloved club would struggle to deal with Liverpool as a result.
Video: Scholes' rant about Man Utd after beating Atalanta
Scholes said: "I get the United excitement, but that first half put me off the second half and if United were playing a better team, they'd be out of sight. They played an Atalanta team who had five players missing and a defender who went off at half-time. I get the excitement, but there was something about that first half that was worrying. It lacked unity and that was a worrying sign.
"Can they play against Liverpool like that? Never in a million years. It's a great night and Cristiano [Ronaldo] is the big hero, but the first half was a real worry for me.
"You celebrate every win, but the first half really worried me. People are going to say I'm miserable and I'll maybe be slagged off by some United fans, but I looked at that game thinking about the Liverpool game. That first half United had two midfielders playing on their own and if they do that against Liverpool and [Manchester] City they'll be four down by half-time.
"Yeah it's great seeing the excitement and attacking. It's brilliant, but that first half has stuck with me. They've come back with great spirit, but they conceded so many chances. If you're playing against quality players, there's no way you can win that game. People get carried away, but will he [Solskjaer] play that way on Sunday?
"Go and do that on Sunday and see what happens. Jurgen Klopp will be rubbing his hands together.
"From a neutral point of view it was great, goals everywhere, excitement - but United can't play like that. I'm being more realistic as to what's to come. Relying on the goalkeeper again, you can't rely on that. They're playing a decent team here, but a centre-back had to go off injured and they had five players out. You can't get too carried away.
"The quality and individual brilliance they have has never been in doubt. Look at this game and you will think they're cured, but they're not. I bet they gave away as much as they did against Leicester.
"Ole was emotional, but the shape was all wrong in the first half. I thought they [Atalanta] had too many chances and, to be fair, when a team is getting beat 2-0 at half-time at home I wouldn't say the performance could be that good. Like I said, he's emotional. He's just won a game, but I didn't like the first half and it's worrying me. If we play like that against a proper team, it'll be disastrous.
"He said before the game 4-2-4 and it was exactly what it was. If they play like that they'll get destroyed. There was no connection between the team. Fred gets pushed out and [Scott] McTominay doesn't know where to go. If that happened could you imagine [Kevin] de Bruyne playing against that? That was brilliant for a fan, but remember who you're playing against! You're playing against Atalanta. Yeah they beat them, but beat them at the end.
"It'll be interesting to see what that does on Sunday. Will they get carried away with the emotion by beating a low-ranked team in the Champions League and go with that on Sunday? We'll see."
Scholes was bang on the money.
United will be in the headlines for all the wrong reasons over the next couple of days, with Solskjaer's position as manager of the club now under enormous threat.