Liverpool re-established their 13-point lead at the top of the English Premier League - having gone a year unbeaten after the 2-0 victory over Sheffield United - but it came at a cost.
The Reds lost midfielder Naby Keita to a groin injury in the warm-up which leaves Jurgen Klopp with just 12 senior outfield players, plus new signing Takumi Minamino, and a number of youth teamers for Sunday's Merseyside derby FA Cup tie at home to Everton.
Liverpool have played 33 matches so far this season, 11 of which have come in the last 30 days, and it is beginning to take its toll on the league leaders - who equalled the best start to a season by any top-flight team with their 19th victory in 20 games.
Keita joined defenders Joel Matip, Dejan Lovren and Nathaniel Clyne, midfielders Alex Oxlade-Chamberlain, Xherdan Shaqiri, Fabinho and forward Rhian Brewster on the sidelines after injuring himself in the pre-match warm-up.
"I don't know exactly what he has, it was his groin, but I can say he won't be involved against Everton," said Klopp.
"That makes it 12 senior players we have, plus the kids. That's not cool.
"I have no clue how it happened, I think it was the last shot during the warm-up, he came in limping."
An early goal from Mohamed Salah was backed up by one from Sadio Mane midway through the second half in a match which was at times looked laboured despite Liverpool racking up the most passes - 874 - they have ever made in a Premier League match.
"We can do things better and we have to. But I saw some things tonight that we did better than the last game, for example," Klopp said.
"There was no party after the game. It's really an intense period of the year, so it's not like somebody wants to go out tonight and have a few drinks, they're all happy when they're in bed - so am I, by the way.
"The players know they want to do better. But for tonight, our performance was as good as anything."
United never got close to their best and their manager Chris Wilder admits they got off lightly with a 2-0 defeat.
Australian Associated Press