Rafa Benitez says Ciaran Clark's going nowhere amid reported interest from Aston Villa.
The defender has been linked with a loan return to Villa Park.
However, United manager Rafa Benitez will not let Clark – who started against Tottenham Hotspur last weekend – leave the club, though Henri Saivet, Achraf Lazaar and Rolando Aarons are available for loan.
"We have some players that need time on the pitch," said Benitez, who signed Clark from Villa two years ago.
"But players playing already, we cannot allow anyone to go."
Benitez lost Florian Lejeune to a knee injury last month.
The defender has had a successful operation and could return later this season.
Meanwhile, DeAndre Yedlin – who suffered a knee injury last weekend – will not be involved against Cardiff City tomorrow.
The defender could return against Chelsea at St James's Park on August 26.
Benitez said: "I think it (the Cardiff game) is too soon. He will stay here. He will need some time to get back to his level."