Sir Alex Ferguson is hoping that Darren Fletcher can make a full recovery, but is not putting a timescale on the midfielder's return.
The Premier League champions revealed earlier this week that the Scottish international is to take an extended break from football as he is suffering from a chronic inflammatory bowel condition.
The Scotland captain missed two months towards the end of last season due to the illness and also sat out United's pre-season tour to North America to work on his fitness.
Ferguson believes the right decision has been made but also acknowledged that his loss will be a blow to Manchester United.
"The medical people have made the right decision. They regarded Darren's health as the more important issue and we as a club agree with that," said Ferguson.
"He's a loss - he's such a great professional and it's not easy to replace that, but we'll think about that. Hopefully he'll make a full recovery.
"It's obviously going to take a bit of time and rest is the most important thing at the moment. I can't put a timescale on it."
The Red Devils travel to QPR on Sunday.
