Greater Western Sydney forward Jonathon Patton has torn the ACL in his right knee for the second time.

The 21-year-old GIANT was injured when he landed awkwardly from a marking contest in the second quarter of Sunday's game against Melbourne at the MCG.

He left the ground immediately and was sent for scans, which confirmed the club's worst fears.

Patton previously injured his knee in round three, 2013 but looked to have made a successful comeback this year, playing 19 games and kicking 25 goals.

The power forward was nominated for this year's NAB AFL Rising Star Award after an impressive performance against Geelong in round 18.

His teammate Dylan Shiel said Patton had been terrific in 2014.

"He has had some pretty big setbacks in his first three years but I'm sure he will rehab his knee the way he always has and that is at 100 per cent and he will be back as good as ever," Shiel said.

Patton clutched his knee in pain after the incident and took several moments to compose himself.

He walked to the bench and went straight down to the rooms to be examined.

He reappeared briefly to run along the boundary but then went back down to the rooms.

"He has had such a bad run with injuries and this year he has been so good for us and he is really leading the big man stakes," coach Leon Cameron said after the game, before the extent of the damage was known.

Patton was the No.1 selection in the 2011 NAB AFL Draft.