STAR forward Steve Johnson looks certain to miss Greater Western Sydney's clash with Port Adelaide on Saturday with a knee problem.

Johnson was forced to sit out the Giants' main training session on Wednesday morning after pulling up sore from his side's round two win over North Melbourne, instead throwing on his coach's hat to help out fellow goalkickers like Toby Greene.

Coach Leon Cameron said the veteran had struggled to get up for the Giants' trip to Tasmania last week, and while he managed to recover in time to face the Kangaroos, Johnson's knee blew up again after the match, and his chances of boarding the plane to Canberra to take on the Power are slim. 

"These older guys, we give them every opportunity to get up, and we'll probably give him until Friday, but if you were a betting man, you'd probably say he's more unlikely (to play) than likely at this stage," he said. 

"I don't want to send any players out that aren't 100 per cent fit, especially against a side that approaches every contest in a manner that they do. 

"With some of these guys that have been around for a long time, you actually want to give them some continuity, because it's hard for them to get up, then have a week off, then play again. 

"So it’s disappointing but that’s where it's at, Steve's had an interrupted pre-season and he missed the last game of the JLT Community Series because he got concussed, so we feel as though we were getting him into a bit of a rhythm." 

While one GWS forward looks set to be forced out this week, Cameron confirmed that Jonathon Patton would return to the senior side for his first game since round one.

Patton copped a knock to the hip against Adelaide in the season opener and was a late withdrawal from the Giants' 102-point win over Gold Coast in round two, but got through a full game in the NEAFL last week to prove his fitness and will give the Giants three tall targets inside 50 alongside Jeremy Cameron and Rory Lobb.

"There's now doubt against different sides you can go in with two (tall forwards), and over the last two weeks it's probably helped us," Cameron said.

"But I'm still a big believer that three can work, as long as they bring some good defensive pressure, and that one area that Jeremy (Cameron), 'Lobby' (Rory Lobb) and Jon have really worked on over the summer period, and I'm looking forward to seeing what they can bring on Saturday." 

Apart from the likely loss of Johnson, Cameron also needs to find a replacement for Ryan Griffen, who suffered a serious ankle injury against North and will miss the next 8-10 weeks of footy. 

The club's No.5 pick at last year's draft, Will Setterfield, would have come under consideration, but he went down with the same injury in the NEAFL on Saturday.

It leaves Matthew Kennedy, Harry Perryman and Harrison Himmelberg as the most likely candidates to come in for Griffen, with the trio of GWS academy graduates combining for 97 possessions and 26 tackles last week in the reserves.