Mourinho was left upset by United's failure to secure a win and said his attackers were to blame at the Stade Constant Vanden Stock.
"The result is positive. To play at home with an advantage is a good result in any case," he told a news conference.
"But I think the result should be much better and we can only blame ourselves because we had an easy game to play in the second half and we didn't play in attack with that instinct and we lost the ball easily one too many times.
"We stopped our counter-attacks because of sloppy decisions and then we gave them opportunity to have some counter-attacks.
"Luckily, our defenders were very much in the game and resolved some of our problems.
"But if I was a United defender I would be very upset with the attacking players because they could have killed the game and they didn't."
The draw is still a positive result for United heading into the return leg at Old Trafford on April 20.
Mourinho said he was happy with some aspects of his side's play, but he lamented their decision making.
"I'd say it was a strong performance by the team from the defensive point of view. [Jesse] Lingard, Mkhitaryan and [Marcus] Rashford, they did defensively what we asked them to do, to compact the team, to control the opponents, to cover some spaces," he said.
"We were very solid defensively, and that is not just about the defenders, it is about the team as a team.
"But then, when the team is broken because we have to counter-attack, we cannot lose so many balls. It is not just about the chances we miss, because we didn't miss so many chances, it's about half chances, the chances where you feel the space is there, the time is there, but then the decision is wrong."