Mourinho praised the Man United defence for their work on the night, but criticised Marcus Rashford, Jesse Lingard, Zlatan Ibrahimovic and Anthony Martial for failing to make the most of their chances.
The Portuguese coach told BT Sport after the game at the Constant Vanden Stock Stadium: "There was lots of space to kill, good chances. Lots of chances that we don't get because of a bad touch, sloppy touch, a flick, bad decisions in the last third.
"Put the performance of two or three of our attacking players together and you squeeze not much juice out of it. Rashford, Lingard, Ibrahimovic, Martial. They were very similar.
"But the people behind very solid very focused. The defenders did the serious work but the people who had to kill the game didn't. We start the game at Old Trafford in front, but we had all the conditions to kill the game."
The return leg takes place on Thursday, 20 April at Old Trafford.