Ole Gunnar Solskjaer hailed Brandon Williams as "exceptional" after Manchester United's Europa League win at Partizan Belgrade, but he acknowledged the side's overall display was far from convincing.
Anthony Martial converted a 43rd-minute penalty, won by teenage wing-back Williams, as United prevailed 1-0 to end an 11-match winless away run stretching back to March.
The Red Devils now hold seven points from three games in Group L and they have yet to concede a goal in the Europa League this season.
Yet Martial's penalty was United's only shot on target in Belgrade and they offered virtually nothing in an attacking sense after the interval as Partizan pushed for an equaliser.
"We got a result, but of course the performance wasn't what we really hoped for," Solskjaer, who made seven changes to the team that drew 1-1 with Premier League leaders Liverpool on Sunday, told BT Sport.
"It would have been nice to have it in a more assertive way. It was won by loads of good defending in the box, but we let them come to our box too easy.
"[There were] some great blocks, some good defending in the box, another clean sheet in the tournament, which is positive, and Brandon was absolutely exceptional."
In an interview with MUTV, Solskjaer added: "In the second half, they [Partizan] made us defend too deep. I'm not happy with the defending at the front.
"That's been one of our fortes and strengths this season, [but] I thought that was non-existent second half and that needs sorting out because we don't want to be a team that is just defending inside our box."
Teenager Williams earned United's spot-kick when he surged past Nemanja Miletic and was then brought down by the Partizan full-back's desperate challenge.
However, the hosts felt they should have had a penalty of their own in the second period when a header from Miletic appeared to strike Williams on the arm, sparking furious protests from the former that resulted in a yellow card.
Williams was one of two teenagers in United's XI, with James Garner handed a first start in midfield.
"I thought Jimmy grew into the game, better and better. We gave him some difficult passes in the first half, in difficult circumstances, but he handled it well," said Solskjaer.
"Brandon, for me, was the man of the match. He was absolutely excellent and I'm so pleased."