Liverpool re-established their 13-point lead at the top of the 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 – three of whom were on the bench against the Blades – 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.

"Everybody who has played football has had this kind of injury, it's not nice. It's painful. I don't know how long it will take, but he won't be ready for Sunday."

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.

"There is space for improvement. How you deal with it in the headlines I couldn't care less. We don't see it as a threat for other teams. For us, it's important that we just care about us," Klopp added.

"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.

"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. It was just really good. That makes me quite happy."

United never got close to their best and their manager Chris Wilder admits they got off lightly with a 2-0 defeat.

"From our point of view I was disappointed because I don't think we laid a glove on them. I think the scoreline flattered us," he said.

"I thought Dean Henderson (the goalkeeper) was the difference between it being three, four or five.

"When you come here you have to try to get a foothold in the game and we couldn't have got off to a worse start. It gave them a big lift and then I thought it was comfortable for Liverpool.

"If there is ever an example for young players – and our players – as to why they are world champions, why they are European champions and look like being Premier League winners as well it is the basic stuff they had to do tonight which they did miles better than us.

"We had to do that a lot better tonight than we did to stand a chance."