Manchester City defender John Stones has been ruled out for up to two weeks with a hamstring injury suffered on England duty.
“I don’t know when he will come back but I think maybe 10 days, two weeks,” said City boss Pep Guardiola.
Second-placed City host fifth-placed United in the Premier League at Etihad Stadium at 14:00 BST.
City also play Southampton in the league and FC Copenhagen twice in the Champions League before facing Liverpool on 16 October.
Midfielder Kalvin Phillips, who has made only one appearance for City since joining from Leeds this summer, had shoulder surgery during the international break.
The England squad for the World Cup is due to be named on 20 October.
“He feels good. He is getting better,” said Guardiola. “He took the right decision.
“Hopefully he can be back in time to go to the World Cup.”