Harnwell has suffered a medial collateral ligament (MCL) tear in his left knee.

The injury was sustained late in the opening half of the club's 4-2 win against Brisbane in what seemed to be an innocuous incident as Harnwell and Roar defender Bob Malcolm were challenging for a loose ball.

Harnwell felt the knock to his knee but there was no pain and after receiving strapping at half time continued to play until being substituted in the 59th minute for tactical reasons.

A follow-up assessment post-match and subsequent scans on Monday night in Brisbane revealed the full extent of the injury which was assessed as a high grade MCL tear and a recovery period of approximately two months.

"We didn't really expect it to be this bad because there was no pain and the knee didn't appear to have too much extra movement in it," Harnwell said.

"I knew something wasn't right after the tackle had been made but there weren't any tell tale signs said I should stop and I felt quite confident I could play on.

"It was a bit of shock when I was told by the physio this morning.  I didn't expect to get the diagnosis that came through and I was really disappointed."

Sunday's match was Harnwell's first start of the season and his second game back for Perth after recovering from a hamstring tear sustained in pre-season.

Playing up front, the big attacker was involved in Glory's opening two goals.  His pressure on defender Josh McCloughlan forcing the Roar man to head into his own net in the first minute while a delicate pass to team mate Wayne Srhoj opened up space for the midfielder to chip past Liam Reddy for Perth's second.

"I was enjoying it out there, I don't think many people expected me to get the start and I wanted to make sure that I made the most of the opportunity," said Harnwell.

"I had worked hard to get fit and force my way back into the side and I guess that just starts again which is not ideal.

"I have been around long enough to deal with it in the best possible frame of mind but it has been a bit of a rollercoaster ride this season.

"It has been the worst run of my career in terms of injury and the most frustrating thing is they have been freak injuries which have had nothing to do with my conditioning."

Harnwell will see the club's doctor on his return to Perth tomorrow and have a rehabilitation programme mapped out.  The 32 year-old has already sought the advice of team mate Jamie Coyne who suffered a similar injury in the club's round one match against Adelaide and is back playing his best football.

Harnwell added: "Coyney was running pretty quickly and I'm hopeful of being able to do that as well so I don't get too far behind in my fitness.  The straight line stuff isn't an issue.

"The most frustrating part is that I did so much work to get back into the team and now I'll just have to focus ahead eight weeks and do it all again.

"I guess there is some consolation in the fact that there are more games this season and I can come back with still plenty to play, however I wanted to be in a position where I was out there helping the team make the finals, not watching from the sidelines."