The Red Devils beat Norwich 1-0 at Carrow Road on Saturday and are due to be quickly back in action once again at Brentford on Tuesday.
However, according to SunSport, United have now had positive lateral flow tests returned on a number of players and staff.
It's reported that the entire squad who travelled to Norwich tested negative beforehand, but there have since been a small number of positive tests among players and staff on Sunday.
As a result, they had to be sent home from the Carrington training ground.
The remaining players trained outdoors and could not train as normal, with sessions adjusted to be individual and non-contact.
It's understood that the Premier League have been informed, but the status of the Brentford match is currently unclear.
The Sun's United correspondent Neil Custis, who broke the story, joined talkSPORT on Sunday and said: "My understanding is that prior to the Norwich match, the whole camp that went to Norwich tested negative.
"But since then, today at Carrington, a small number of players and staff have tested positive and been sent home.
"Training's had to be altered to players training individually and no contact being made because obviously more positive tests could come.
"This puts in jeopardy Tuesday's game at Brentford.
"We've seen what happened with Tottenham."
According to The Athletic, Aston Villa have also suffered an outbreak and cancelled training on Sunday as a result.
Of their cases, it's reported that only one is a player.
As such, they are likely to train as normal on Monday and their match against Norwich on Tuesday is expected to go ahead as planned.