It is the second hamstring problem 24-year-old Pogba has suffered in six months.
"I don't know (what the severity is)", Mourinho told MUTV, the club's in-house television station.
"I feel pretty confident and know what I'm able to do".
"It looks for me too [like a hamstring injury] but I haven't spoken yet to the medical department".
Manchester United boss Jose Mourinho says he is "disappointed" with Andreas Pereira's decision to join Valencia on loan on deadline day.
Rashford, who made his Champions League debut on Tuesday, said Pogba's loss was a big blow.
"But Paul's going to be a massive miss and hopefully we can get him back fit as soon as possible". Everton have one win from four games. If we want to be champions, then we have to win (those) types of games (like Sunday's).
'But the nature of the Premier League is also changing and we need defenders who can play, who can come into the midfield bringing the ball because it's becoming really defensive and we're going to face many teams with five in the back and two or three in front'.
The teenager said United's strong squad would help them cope with the loss of Pogba. The teenager believes United's embarrassment of riches is helping rather than hindering his and Martial's development.
"He's a very important player for me, much more important than what you can imagine. But was his decision, I opened the door, he has a long-term contract, we have the option to bring him back on January 1, so is not a drama but is a decision which disappoints me a little bit".