The Frenchman limped out of United's 3-0 Champions League win over Basel on Tuesday with a hamstring injury.
Starsport understands Pogba also injured his thigh, which may lead to a longer period on the sidelines.
But boss Mourinho says claims his star midfielder will be missing for three months are nonsense.
Speaking after his side beat Everton 4-0, Mourinho said: "I don't know if it is 12 weeks or 12 days, honestly.
"Any comment or any new rumour is totally wrong because we don't know if it is 12 weeks or 12 days.
"The player was diagnosed initially after the match because of the conditions of the muscle and the bleeding.
"The decision was one more week to wait to see really clearly in the scans, that I mentioned, of the injury."
And Mourinho has admitted the club will wait before performing more tests on the 23-year-old.
"Nobody in this club spoke about 12 days or 12 weeks. Not at all," he added.
"In the hamstring, it's a normal procedure and we wait a few more days.
"So it's completely nonsense information."
United are fairing well without their £89m man, looking comfortable as they rejoined Manchester City at the top of the Premier League table this evening.
The Red Devils take on Burton in midweek before travelling to St. Mary's on Saturday to face Southampton.
