GREATER Western Sydney youngster Paul Ahern will spend a second straight season on the sidelines after tearing the anterior cruciate ligament in his right knee for the second time this year.

Ahern hasn't played a game at any level this season after rupturing his ACL in a training drill in February.

The 20-year-old suffered the same fate on Wednesday morning, as the Giants trained behind closed doors at ANZ Stadium in preparation for their qualifying final clash with the Sydney Swans this week. 

It's believed the injury occurred during a standard training exercise, but scans revealed the worst possible news.

The talented midfielder, who was the club's seventh selection at the 2014 NAB AFL Draft, has only recently rejoined the main training group at GWS, with an eye on working towards his senior debut next season.

The loss of Ahern comes just 24 hours after the Giants lost Jack Steele for the rest of the season with a foot injury, also suffered at training, on Tuesday.