The 32-year-old Schweinsteiger, who announced his retirement from international football last month, is yet to make a first team appearance this season and has been training with the club's reserves.
"It is very difficult to happen. I'm not saying it's impossible, I'm saying that it's very difficult. We have five players for two positions. It's very difficult that an opportunity will arrive (for him)," Mourinho told reporters.
Mourinho is surprised Bayern Munich have not attempted to bring Schweinsteiger back to the club where he played for 13 seasons before joining United last year.
Bayern chairman Karl-Heinz Rummenigge said players were going to think long and hard before joining United following Mourinho's treatment of Schweinsteiger.
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"I thought, when I read some quotes from people at Bayern Munich, they would run to Manchester to bring him back, but no, that didn't happen. I'm quite surprised Karl-Heinz Rummenigge is not here to take him back," Mourinho added.
The 53-year-old Portuguese does not expect the top teams to falter like they did last season and believes clubs vying for the title would need more than the 81 points Leicester City collected to win the Premier League this year.
"I think that more points will be needed to win the competition because many of the top teams under-performed last year, which was amazing for the beauty of the competition," Mourinho added.
The former Chelsea boss also confirmed that United were not planning on bringing in more players in the current transfer window.
"So I think we're going to have a very quiet week, not surrounded by helicopters, waiting for somebody to leave or to arrive, so I am more than happy with the squad I have," he said.
Three-times European champions United, who have been drawn in a tough Europa League group alongside Fenerbahce, Feyenoord and Zorya Luhansk, travel to Hull City on Saturday.
(Reporting by Ian Rodricks in Bengaluru, editing by Ed Osmond)
