Wesley Hoedt headed into the back of Lukaku's head during an aerial challenge and the Reds' top scorer lay flat out on the turf for some time, receiving extensive treatment. Marcus Rashford replaced the Belgian and there was genuine concern among the crowd for the centre-forward, whose injury prompted six minutes of stoppage time at the end of the half.
"That's football," Mourinho told MUTV. "He was just unlucky. I think the Dutch central defender goes with him for the ball and it's unlucky for Romelu and unlucky for Hoedt. It was nothing aggressive and, hopefully, nothing important. But the first consequence of is he didn't play for the rest of the game."
United's next outing is away to Lukaku's former club, Everton, on New Year's Day, with Derby County to follow in the Emirates FA Cup on Friday, with the possibility the striker will miss both fixtures.
"I don't know," Mourinho added about a timescale on the summer signing's return. "I know that normally bad, bad news arrives immediately. Bad news didn't arrive. I think that there's a problem for the next two matches. For sure."