United conceded from another set-piece.
"When we're talking about set pieces I think we can be over analytical these days and go into so much detail until we're almost blue in the face. Ultimately, go and attack the ball, be aggressive and get first contact on the ball. United didn't do that enough today," said Phil Jones.
Former Manchester United defender Jones has made a strong start to working as a pundit and his analysis for Nottingham Forest's first goal at the weekend was simple but spot on.
United have conceded three goals from set-pieces in their last two games in the Premier League and a lack of aggression was one of the reasons for Nottingham Forest's goal on Saturday.
There was also a lack of organisation and communication. Diogo Dalot's positioning was poor, Lisandro Martinez was marking the 6ft 4in Nikola Milenkovic and Manuel Ugarte was trying to mark Chris Wood. There were mismatches galore and it wasn't a surprise that Forest scored.
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Both of Arsenal's goals in their 2-0 victory over United at The Emirates came from corners. The deliveries from Declan Rice and Bukayo Saka were deadly but the goals were avoidable.
Suddenly, United have a weakness that teams are bound to target and Ruben Amorim will have to brush up on the defending of corners to prevent it from causing trouble again.
Only Wolves and Southampton have conceded more goals from set-pieces than United this season and it is clearly something that needs to be worked on by Amorim's coaching staff.
Andreas Georgson was appointed as United's set-piece coach in the summer and was retained as part of Amorim's backroom staff, despite the bulk of Erik ten Hag's coaches departing, but it was recently confirmed that Carlos Fernandes has taken control of coaching set-pieces.
Amorim was asked about the change and clarified: "The thinking is Carlos is responsible for set pieces, different stuff. Andreas is also there to help. That’s it. That’s the reason."
With the power of two brains on set-pieces, United have no excuses to be among the worst in the league at defending such scenarios but there is only so much coaches can do.
Maguire replaced De Ligt on Saturday.
At the end of the day, the players are responsible when they cross the white line on a matchday and need to show more desire and aggression to improve the set-piece record.
You can't coach aggression and sometimes having an aerial presence is innate. The best player United have at defending corners is Harry Maguire. He is often imperious in aerial duels and it makes sense to bring him back into the starting team at a time when set-pieces are a big issue.
Maguire was brought on for Matthijs de Ligt after the hour mark against Forest and improved the physical presence. "We put Harry Maguire on because of set-pieces," explained Amorim.
"To control more who was winning all the first balls, the only way to attack Nottingham was transitions, to try and win first and second balls. When Harry was on the pitch he won all the first ball."
United have two games against FC Viktoria Plzen and Manchester City this week and the latter is the fixture that matters. Maguire seems guaranteed to start in at least one of the matches but it's logical to start him in both and bring him off to manage his load in the Europa League.
Perhaps Maguire should start against Viktoria Plzen on Thursday night and be substituted after the hour mark. That would give United the best of both worlds: Maguire's presence for their Europa League fixture and a player who was relatively fresh for the Manchester derby.
Maguire has been slightly forgotten in recent weeks but was in fine form before his injury and it would be foolish not to lean on his quality when the team is struggling from set-pieces.
He is not a fashionable choice to start for United but he is a player you want in the trenches.