Ole Gunnar Solskjaer admitted he is unsure if Harry Maguire will be available for the Manchester derby after missing United's FA Cup tie.
Harry Maguire has emerged as a concern for Manchester United ahead of Sunday's derby with Manchester City due to an ankle injury.
The England international was absent for United's FA Cup fifth-round tie at Derby County on Thursday having rolled his ankle in training.
Ole Gunnar Solskjaer revealed he had intended to play United's captain against the Rams on his 27th birthday.
However, Maguire's injury has left United boss Solskjaer unsure over whether he can pick the centre-back for Sunday's clash with Pep Guardiola's City at Old Trafford.
"I had a word with him before training yesterday, that I am not going to rest him [against Derby]," Solskjaer told MUTV.
"But then he rolled his ankle in training so he had to stay at home. Hopefully he will be okay for the weekend, but I am not sure."
City have their own injury concern over a key man after Kevin De Bruyne missed their FA Cup win over Sheffield Wednesday with a back complaint.
Solskjaer made six changes from the side which drew 1-1 at Everton for the game against Derby, with Juan Mata captaining a team that featured Odion Ighalo.
United's all-time record scorer Wayne Rooney was named in Championship side Derby's starting XI.