Scans have revealed a grade one tear of the medial collateral ligament (MCL) in his left knee.

The 21 year-old suffered the injury during the first half of Perth's National Youth League match at ME Bank Stadium on Saturday afternoon and had to be stretchered from the ground raising fears he could face an extended period out of the game.

"It's a better result than we initially expected.  The pain he felt when the injury first occurred was quite severe so this is a good outcome all things considered," said Perth Glory physio Chris Hutchinson.

"Jimmy will have some rest and light bike work this week before stepping it up to running the week after and then back into training.

"If all goes well and we're able to stick to the rehabilitation timeline then I expect that he will be able to play again within four weeks."

Downey hasn't played an A-League match this season after sustaining a leg injury in pre-season but was close to a recall to the senior side after three consecutive matches with the youth team.