Young, 27, missed out on the England squad for the World Cup 2014 qualifying double-header against Moldova and Ukraine earlier this month, and Sir Alex Ferguson now expects to be without the player until October.
Ferguson confirmed the problem at his pre-match press conference ahead of Sunday's game against Liverpool.
The United boss also said midfielder Darren Fletcher would play from the start in Wednesday night's Capital One Cup tie at home to Newcastle.
The Scotland international came on as a substitute in the 1-0 Champions League win over Galatasaray in midweek, his first competitive outing since last November. The player has an inflammatory bowel condition.
Ferguson also played down suggestions that Rooney could be able to return in time for Sunday's game against Liverpool, though his recovery from a cut leg is progressing.
“We have everyone available who was available last Wednesday,” Ferguson said. “I know there were reports in the paper about Rooney being possible, but that is not the case. He's not far away, I think he may be fine for next week.
“He's doing a lot of great training and has been working hard. It's just a matter of getting him into the full football training side of things, which he has done part of.
“Ashley Young has got a bruised cartilage - I think it's going to take two or three weeks more. We're just taking our time with that one. You can easily play with it and train but we're not going down that road, we want to make sure he's okay.
“We get a lot of experiences these days of these injuries reoccurring when they come back too quickly and we don't want that with the players - we want to give them a full recovery. He should be okay with two or three weeks’ time.”
- Sir Alex Ferguson
- Darren Fletcher