Costa has a suspected torn anterior cruciate ligament (ACL), which he sustained in the second half of The Reds' National Youth League match against Melbourne Victory at Hindmarsh Stadium last Friday.

Having had scans yesterday, the 17 year-old visited a surgeon today and will undergo surgery tomorrow afternoon, which will determine whether he requires a full knee reconstruction and confirm the worst possible news that will rule him out for six to12 months.

"He (Costa) has been assessed by specialists as having a left ACL tear, which was confirmed by the MRI scan," Adelaide United Director of Football Michael Petrillo said.

"He will be having an arthroscopy tomorrow and if that confirms the tear, they will operate straight away with a full knee reconstruction on the ACL."

If it is confirmed that Costa does need to undergo a full knee reconstruction, Petrillo said the club would look into bringing in a replacement underage player.

"I think we'll be looking to bring in an injury replacement player. Unless we don't believe that the person will add any value to the squad, we'll stay as we are but we will look at the possible options."

Costa's setback sees him join defender Michael Marrone and Paul Reid on the sidelines, Marrone and Reid still expected to be up to three weeks from resuming.