The all action midfielder suffered the problem during his side's loss against Southampton in the Premier League.
While Mourinho is not sure when Sissoko will be back, he did confirm that he suffered a problem in the game against the Saints.
"I don't know. I don't know but injured from the game against Southampton," Mourinho told reporters.
"A ball he had with (James) Ward-Prowse. He had it and felt it but kept playing until the end.