Belgium midfielder Fellaini suffered a knee injury while on international duty last week and could be out for at least a fortnight.
The 29-year-old had been in good form and scored twice in United’s last Premier League outing, their 4-0 win over Crystal Palace, on September 30.
Asked about the injury, Mourinho told Sky Sports News: “No blow, no problem. We don’t cry with injuries. We may cry with the process that leads to the injuries but we don’t cry with the injuries. We just trust the players that are going to play.”
Asked if that meant he had concerns about players going away with their international teams, he said: “Not just international teams, we don’t only have injuries in international teams.”
United have also had concerns about the fitness of striker Romelu Lukaku but he came off the bench and scored in Belgium’s 4-0 win over Cyprus on Tuesday.
The 24-year-old, who has scored 11 goals for United since his summer move from Everton, missed the previous game against Bosnia with an ankle problem.
Southampton remain in the hunt for Red Bull Salzburg's Amadou Haidara and are reportedly readying a £15million offer.
Aston Villa are reportedly weighing up a summer move for out-of-favour Huddersfield defender Michael Hefele.
Southampton are lining up a summer bid for Huddersfield Town midfielder Aaron Mooy, reports claim.
Rangers manager Steven Gerrard is weighing up a move for Liverpool full-back Jon Flanagan, reports claim.
Scottish champions Celtic are reportedly ready to offer full-back Calvin Miller a new deal before sending him out on loan.
Everton target Jack Wilshere has sparked a scramble for his signature after confirming he is leaving Arsenal this summer.