Defender Sean McGinty has returned to Manchester United after his loan spell at Oxford United was terminated.
McGinty, 18, was originally due to stay with the U's until January, but has not played a minute of league football with Chris Wilder's side this season.
The U's hope to sign a replacement by Saturday's game at Burton.
"We've made a decision to send Sean back and I've explained that decision to Manchester United," Wilder told BBC Radio Oxford.
"He's gone back and we'll be looking to fill that position. They understand the reasons why he has gone back.
"There are a couple of players we are looking at.
"But this club has made a stance regarding short-term loans so we are looking to bring someone in possibly until January.
"The window is not the biggest, but we've looked and hopefully we can get something sorted before the weekend because it is important we get that position covered."