Jose Mourinho has confirmed that Nemanja Matic will miss the start of the Premier League season after undergoing surgery.
The midfielder sustained an abdominal injury during the World Cup, as Serbia were knocked out in the group stages.
And Mourinho has now revealed that Matic has been forced to go under the knife, meaning he faces a prolonged spell on the sidelines.
"Matic arrived from the World Cup, holiday, direct to surgery because something happened in the World Cup," Mourinho told the club's official website.
"He has had surgery. He is out for the start of the season. He came from the World Cup with the injury and the time he had to rest on holiday was not enough to resolve the problem.
"I don't know how long he will be out for."
United were beaten 4-1 by LIverpool in a pre-season friendly on Saturday, and Mourinho has bemoaned the thin nature of his current squad.
"We play Real Madrid and we have no more players than what we have," he told MUTV. "Matic arrived from the World Cup, holiday, direct to surgery because something happened in the World Cup. Antonio Valencia comes from holiday - I think too much holiday for him. His condition was not good when he was back, then injury and also go back.
"Anthony Martial has the baby and after the baby is born - beautiful baby, full of health, thank God - he should be here and he is not here. So we have Alexis (Sanchez) and (Juan) Mata who play every minute of every match and now we have Real Madrid and after that Bayern Munich and that's our start.
"I just hope the boys on deserved holidays, they take care of themselves a little bit and I hope somebody wants to do what (Marcus) Rashford and (Phil) Jones decide to do, which is to be back a little bit earlier to try and help the team because for the beginning of the season we are going to be in trouble.
"The majority of the players that played are not going to play - some of them are not even going to belong to the squad."
United face Leicester City in their opening Premier League game on August 10.