Ole Gunnar Solskjaer was delighted with a resolute performance from Manchester United in the 1-0 win over Partizan Belgrade on Thursday.
Brandon Williams and 18-year-old midfielder James Garner were among seven changes made by United's Norwegian boss from the side that started Sunday's draw with Liverpool.
Anthony Martial scored the only goal of the game on his return from injury.
The Frenchman was cool, calm and collected when he converted from the spot after 43 minutes, following a challenge on Williams in the box.
Speaking after the game, Solskjaer said: "Second half I thought we dug in really well in the box, defended really well in the box.
"They played well second half made us defend too deep I'm not happy with the defending at the front. I think that's been one of our strengths this season but I thought that was non existent in the second half.
"That needs sorting out because we don't want to be a team who is just defending in the box."