Mourinho would not elaborate on suggestions that Pogba had aggravated a long-term hamstring problem, or claims that the midfielder has been working with a personal trainer against club advice.
"After 2-0 I think everything changed and we stopped to play, stopped to think, stopped to play seriously, ' Mourinho said, as quoted by the Daily Mail". So for me, it's simple and objective that he's not playing this weekend. I don't know that I just it is a muscular injury.
Pogba is set to miss United's league games against Everton, Southampton and Crystal Palace, plus next Wednesday's third-round League Cup tie against Burton and the Champions League trip to CSKA Moscow on September 27.
"But there are good players more than ready to play so honestly we miss Pogba, we need Pogba but we have good players".
Pogba, 24, left Old Trafford on crutches - and scans have confirmed United's worst fears.
He added: "So if no Paul for Sunday, we have [Ander] Herrera, we have [Michael] Carrick, we have Fellaini and we have [Nemanja] Matic".
And Mourinho says the England worldwide is available for selection once again, but Sunday's visit of Everton (3:30pm) might be too soon.
However, the Monaco squad played well, and this encouraging performance despite the absence of Lemar may well make them more amenable to an approach in January - with United ready to break the bank to land the player, who was the subject of plenty of transfer attention over the summer from Arsenal and Liverpool.