The highly-rated 19 year-old suffered cruciate knee ligament damage in May and is expected to be sidelined for some time.
Although United have since signed two more forwards in Robin van Persie and Angelo Henriquez, Sir Alex Ferguson is known to hold Keane in high regard and had promised him a place in the first-team squad for this season.
"Will's working very hard in the gym," Under-21s boss Warren Joyce informed ManUtd.com. "He's got to make the most of a bad time really and see how much he can develop his body in the coming months so he comes back even stronger.
"If he does that, he's got something from a really bad piece of luck and benefited from it."
Keane sustained the injury whilst on international duty with England Under-19s at the end of last season, which caused him to miss the European Championship finals. Sam Johnstone, Tom Thorpe and Will's twin Michael helped Noel Blake's side reach the semi-finals before an extra-time exit to Greece.
"They did well as a group but I think, if Keane and Larnell Cole had been fit, they would have gone further than they did in the tournament," added Joyce.