The Portuguese boss, who was under the spotlight following three consecutive defeats last week, witnessed his team bounce back with a 3-1 victory this evening.
Referee Stuart Attwell spotted the incident and issued a yellow card, but Mourinho feels that the match official made the wrong call.
"It was difficult, as I was expecting, but we were in control for the first 35 minutes and the last 35 minutes," Mourinho told Sky Sports News. "The subs (Marcus Rashford and Zlatan Ibrahimovic) were just a contribution.
"The game could and should have been easier. Normally it's easier against 10 men and we should have been playing against 10 men early in the second half, but they fought, they played with their strength.
"I should stop speaking about refereeing decisions and wait for you to speak about them, some with honest vision, some with dishonest vision. It's a dangerous situation. It could have been an important injury."
Meanwhile, United will go head to head with Manchester City in the fourth round of the competition.