ST KILDA'S bid to revive its season has suffered a blow with key forward Josh Bruce ruled out for up to two months with bone stress in his lower back.

Bruce trained on Thursday but pulled up sore and coach Alan Richardson told reporters at Moorabbin on Saturday his side would be without the left-footer for a while.

"Unfortunately for Brucey, his form hasn't been great, but it looks like it's going to be an extended period out now, with a stressy of the back," Richardson said.

"Something like about six to eight weeks, which is disappointing for him and disappointing for us, obviously."

Bruce, 25, booted 124 goals in 2015-17, but a slow start to this season saw him kick just four in his three matches.

He is expected to avoid surgery and will be able to do conditioning work throughout his rehab.

Bruce had been carrying soreness but there had been no major concerns about him playing through that – information Richardson made sure to get from his medical staff.

"You can imagine all those questions get asked, given his form over the last couple of weeks wasn't great," Richardson said.

"My understanding was he'd been a little bit stiff but was really comfortable that he was physically right to go. It clearly went to another level on Thursday (at training)."

Bruce is one of two forced changes from last week's team, with backman Jimmy Webster (hip) to miss Sunday's clash against Geelong at GMHBA Stadium. Tough pair David Armitage and Nathan Wright have been dropped.

Richardson admitted he erred in selecting Armitage, 29, for the round one side and playing him through after the midfielder managed just two games last year amid two bouts of groin surgery.

"He just hasn't been at his best, Dave. He's missed a lot of footy. I've probably brought him back a little bit too early I reckon, in hindsight, wanting him to be that player, needing him to be that player," Richardson said.

"The reality is the game is just too demanding to not be at your best. We found that as a team, let alone for individuals.

"Dave just needs to go back now and get some real run and some real zip in his legs, and be aggressive at the contest again. We need him back in the team playing really strong, aggressive footy."

The Saints are hopeful Webster and Tim Membrey (right knee) will be back the following week for their clash against Greater Western Sydney. However, neither took to the track for the club's main training session last Thursday.

"Both the boys, Tim Membrey as well, ran today and ran pretty strongly, given where they're at with their injuries. We're hoping that we might have both those boys available next week. It depends on, as their intensity steps up at training, whether they can cope with that load," Richardson said.

Small forward Jack Lonie, big-bodied onballer Jack Steele, experienced defender Sam Gilbert and Tom Hickey return to the senior side.

Hickey, who had a right shoulder reconstruction over the off-season, is expected to be the primary ruckman, with rookie Rowan Marshall pushing forward, although Richardson could swap the pair around regularly.

"We think that there's some flexibility there. I actually like Tom as a forward as well. He's done some pretty good work for us in the past, particularly as the deeper forward. He's a good mark of the ball (and) he's a pretty good mover for a big man," Richardson said.