Ole Gunnar Solskjaer admitted he is concerned about Manchester United's defending, especially from set-pieces, after they were beaten at home by Liverpool.
The Red Devils took the lead through a deflected Bruno Fernandes strike but went on to lose 4-2 at Old Trafford on Thursday as Jurgen Klopp's side boosted their Champions League hopes.
Liverpool's equaliser came following a corner, Diogo Jota turning in a Nat Phillips effort, and they took the lead in first-half injury time when Roberto Firmino headed in Trent Alexander-Arnold's free-kick.
Firmino got his second just after half-time from a rebound from a Dean Henderson save and, although United got a goal back through Marcus Rashford, Mohamed Salah struck a fourth with a late breakaway goal.
United's defence rarely looked comfortable without captain Harry Maguire as they suffered a third defeat in eight days in all competitions, while they have now conceded 27 home goals in the league this season, the most in a single season since 1969-70.
More worrying is their persistent fallibility from dead-ball situations. United have conceded 33 per cent of their league goals against in 2020-21 from set-pieces (14 out of 42, not including penalties), which is the highest ratio in the division.
They also conceded from a last-minute corner against Milan in the Europa League last-16 first leg, while Manchester City's goals in their 2-0 EFL Cup semi-final win at Old Trafford came from a free-kick and a poorly cleared corner.
Manager Solskjaer would not criticise his players for the way they lost possession in the build-up to Liverpool's third, but those set-piece concerns are ones he wants to address.
"The goals we conceded, set-plays, [it is] disappointing," said Solskjaer. "That is something we have to work on because we have conceded too many goals from it.
"The third one, we want to play out, we have been good at playing out, we put pressure on them to play out and sometimes we've scored goals from it.
"Today, unfortunately, the two or three mistakes that happened cost us a goal. But they tried to correct it by winning it back and then we lost it after winning it back.
"Football would be really boring if no-one made a mistake. That is the whole idea of football. Of course, you try to eradicate them and work on them, but we have conceded too many goals and we could have avoided some of these and especially the timings of them.
"We're going to use the next couple of days well, to recover physically and mentally, and then we have to prepare for Fulham and then Wolves and then the final. We should have more energy.
"We should use this period well. More energy means a better press; a better press means less chances against.
"We will work on it and brush up on a few details, pay attention to some details on the set plays and put the players in the frame of mind that these set-plays are vital in football. But we can do so much better and when we do that, we will improve."
With Maguire set to miss their final league games against Fulham and Wolves, and with the captain a major doubt for the Europa League final on May 26, Solskjaer is looking for others to take on leadership roles.
He said: "Harry has been amazing since he came to the club, he's been more or less ever-present, so of course, when a player is a leader, a captain, the status of him, his presence is missed.
"But then we've got players who can come in and it's a chance for everyone to take on that little bit of leadership. We need to do that when he's gone.
"You can hear him in the stands when he's not playing. He is important for us and someone else has to just take that responsibility now.
"I'm sure that, in the next two games, we will be without him. Hopefully, we can get him in for the final but who knows? I don't know yet."