Gary Neville said Romelu Lukaku "struggled" in the Manchester derby but Graeme Souness disagreed.
David Silva and Nicolas Otamendi scored goals from set-pieces as Pep Guardiola's side won 2-1 at Old Trafford to move 11 points clear of their opponents at the top of the table and win their 14th straight Premier League match.
Lukaku cut a frustrated figure leading the United forward line and had a hand in both of Man City's goals in Sunday's derby.
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The Belgian failed to clear his lines on two separate occasions which allowed Silva and Otamendi to punish the home side.
Lukaku had a chance to redeem himself late on but his finish from six yards was brilliantly saved by Ederson, meaning he's only scored two goals in his last 12 matches.
Neville thinks the striker looks short on confidence.
"You look at him today and he struggled," Neville said.
"I'm not just talking about struggling in terms of missing a chance, he struggled at being in the game and being confident in the game.
"He's got to get over that quickly as he's Manchester United's centre-forward. He's a young lad but he needs to get to the point where he believes he should be out there - it's like he doesn't believe it in this period."
Lukaku joined United from Everton in the summer for £75million and scored in his first seven Premier League games for the club.
Souness believes that he's currently in a blip in terms of his form and would rather lay the reason for his lack of goals at the poor service provided by his midfielders.
"When you come with a big price there's always pressure but I wouldn't be too critical of him," he said.
"I think it's the team he's playing in. He's not playing with a David Silva or Kevin De Bruyne - he's feeding on poor passes. Sharing balls, balls that are hit at the wrong angle. I think the boy is running around but he's getting no quality service."