Pogba has missed the last four games with a hamstring injury and will sit out Crystal Palace's visit to Old Trafford on Saturday.
Sources have told ESPN FC that the France international could now be ruled out until November.
And speaking at his weekly news conference at Carrington on Friday, Mourinho said: "It's not an injury that I can have the hope I have with [Antonio] Valencia or [Phil] Jones. I have the hope to see them in training.
"Long-term injuries, I don't speak about them. Ibra, Pogba, Rojo, I don't think about these players."
Valencia, Jones, Marouane Fellaini and Michael Carrick also missed the 4-1 Champions League win over CSKA Moscow on Wednesday, while Anthony Martial was forced off in the second half.
United were due to train on Friday afternoon and Mourinho hopes he will have some of them back to face Palace, although Carrick is certain to miss out.
He added: "We have to train and make decisions after training, but I hope some of the players who couldn't play in Moscow, maybe not all of them, but some of them [will be back].
"Some will be out. From all the injuries we had, I hope that I can have some players available."
United face a Palace side who have not won a point or scored a goal in the Premier League this season, but Mourinho warned: "The mentality is very important because we played Wednesday night and we arrived in Manchester at 4 a.m. so home 5 a.m. and we have to play on Saturday, so mentality is very important."