United looked set to take a 1-0 lead into next week's second leg but Leander Dendoncker's thumping header four minutes from time gave the Belgian's an important equaliser.
The away side had ample opportunities to kill the game with goalscorer Henrikh Mkhitaryan and Paul Pogba guilty of misses.
And Mourinho has pinned the blame for United's draw on the shoulders of his forwards.
Jose Mourinho was not happy to draw the game
He told BT Sport: "In my poor English I cannot find a better word [than sloppy]. You have to play more seriously.
"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."
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Mkhitaryan had given United the lead in the first half but they were unable to put the game to bed.
That allowed the Belgian side back into the game and they were punished late on.
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Michael Carrick said: "It's frustrating. We were so in control but got sloppy, we have to kill teams off. We should have won that game. They had one chance and scored a header. We need to be more ruthless, it's as simple as that. We were too comfortable. We got sloppy.
"It's not the end of the world but we are better than that. It's that little edge that makes the difference in the end, but we'll look forward to the next game."