The midfielder went off with knee ligament damage in Belgium’s 4-3 World Cup qualifier win in Bosnia.
But the Manchester United boss isn’t worried because he insists he has adequate cover in his squad.
“I don’t know how he is,” he said.
“From what I read I don’t think he can play the match. It is no problem. Another one plays, no problem for me.
“We do not cry with injuries. We may cry with the process that leads to the injuries but we do not cry because of the injuries.
“We do not have just injuries in international teams.”
The injury is expected to keep Fellaini out for at least two weeks.
The former Everton midfielder has been in impressive form so far this season, scoring four times in his first eight games, including a brace in the 4-0 thrashing of Crystal Palace.
He’s become an important part of Mourinho’s plans, but the Portuguese isn’t worried about players missing through injury.
He added: “We trust everyone. We lost players last season. We lost players this season.
"And [for] our way of thinking and feeling, it's no problem. One player is not playing, another one plays.
"It's an opportunity for another one and we trust. And we never hide behind this or that player, this or that injury.
"We trust the ones that are going to play."
United sit second in the Premier League and are only behind rivals Manchester City on goal difference.
They face seventh-placed Liverpool on Saturday.
