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Schweinsteiger, 32, who won 120 caps for Germany, has trained alone and with the under-23s since Mourinho took charge in the summer.
Schweinsteiger said on Wednesday he "will be ready if the team needs me".
Mourinho said: "We have five players for two positions. It's very difficult that an opportunity will arrive."
World-record signing Paul Pogba and Marouane Fellaini started in midfield for United in last Friday's Premier League win over Southampton, while Morgan Schneiderlin, Ander Herrera and Michael Carrick are also in the squad.
Schweinsteiger, who signed a three-year contract in July 2015 under Mourinho's predecessor Louis van Gaal, has said that United will be his last club in Europe.
Mourinho said on Friday that he had "no problem" if Schweinsteiger chose to stay at the club despite not playing.
"I cannot answer for him. It's his life. It's his career," said Mourinho.
"He has a contract and has the right to make that decision to stay. That's not a problem for us."
Meanwhile, 20-year-old Brazilian midfielder Andreas Pereira will join Spanish club Granada on a season-long loan.
"I love Andreas Pereira as a player and I would love to have him in my squad, but he will go to Granada," said Mourinho.
Manchester United manager Jose Mourinho has said it will be "very difficult" for midfielder Bastian Schweinsteiger to play for the club again.