The two strikers were hurt in the side's goalless draw with Southampton at Old Trafford on Saturday.
Mourinho puts brave face on Ibrahimovic and Lukaku injuries for Manchester United
Zlatan Ibrahimovic has been ruled out for a month and Romelu Lukaku for at least two games as Manchester United's festive frustrations mounted with a goalless Premier League draw against Southampton.
After being held by Leicester and Burnley either side of Christmas, United served up a goalless stalemate against the Saints, with injury problems up front casting an even larger shadow.
Ibrahimovic was a surprise absentee from the matchday squad, while Lukaku was taken off on a stretcher after a nasty collision with Wesley Hoedt.
The Swede recently completed a speedy recovery from a career-threatening cruciate knee ligament injury but Mourinho offered a gloomy assessment of the Swede's apparent relapse.
"Zlatan is out for a month, the same knee. (It's) an incredible problem," he told BBC Sport.
"A 37-year-old man, a lion like he says, a real lion, but it's not easy. Now he had a little recurrence and one month out."
Ibrahimovic actually turned 36 in October, but if the rest of Mourinho's facts were accurate it is bad news for a club who failed to put away a Southampton side who came to Old Trafford winless in their last seven matches.
Lukaku provides another source of worry, missing a golden opportunity to nod his side in front after just three minutes before the clash with Hoedt and a lengthy spell of treatment which ended with him carried away in an oxygen mask.
Mourinho suggested the injury was not a drastic one but does not expect to have the Belgian available against Everton or FA Cup opponents Derby, meaning a heavy workload ahead for Marcus Rashford.
"I know normally bad news arrives immediately and bad news didn't arrive but to leave the pitch the way he did...we didn't think twice to make it (the substitution) immediately. It's because there is a problem, not just for today but the next two matches for sure."