Manchester United interim head coach Ralf Rangnick has revealed that Fred suffered a "muscular issue on his hip flexor" during Saturday's 1-0 loss to Everton.
The Brazil international had to be substituted in the latter stages of the first half at Goodison Park, with Paul Pogba entering the field as his replacement.
Rangnick has said that an issue with the midfielder's hip led to him leaving the field, although it is unclear how long the 29-year-old could be on the sidelines.
Man United were already missing one defensive midfielder against Everton, with Scott McTominay ruled out of the contest late last week due to a foot problem.
"It seems to be some muscular issue on his hip flexor, probably, I don't know exactly what it is, but from the way that he explained it to me it seems to be a muscular problem," Rangnick told MUTV. "After Scott McTominay, missing a second defensive midfielder, let's hope that they will not be out for too long."
Fred will be bidding to return for Man United's next Premier League game against Norwich City on April 16.
