Clarke limped off the training track on Tuesday, but skipper Matthew Pavlich said he was unsure of the exact nature of the injury or how serious it was.
If Clarke is ruled out, it's likely either Hannath or Kepler Bradley will earn selection against the Eagles.
With No.1 ruckman Aaron Sandilands (hamstring) already ruled out of the match, Jon Griffin will take the lead role against Eagles champion Dean Cox.
Clarke was set to play a larger role in attack this year in order to provide support for Pavlich, meaning his replacement could come down to who is the better forward out of Hannath and Bradley.
Hannath was training with the Melbourne Demons over summer when told he had been snared by Fremantle with pick No.8 in the pre-season draft.
The 21-year-old has developed significantly over the past two years while playing with Central Districts, and his 201cm, 102kg frame could sway selection in his favour.
Bradley booted just six goals in 12 games last year, and his waywardness in front of goal remains a concern.
Pavlich said it didn't matter who replaced Clarke - as long as they did their bit for the team.
"Whoever it is, we don't care about the name or the person," Pavlich said on Tuesday.
"If we get them to compete and play to our standards, the rest will take care of itself.
"It's certainly not ideal. We thought Zac competed really well against the Bulldogs (during the pre-season), but what we know is we have some great back-up.
"Kepler played most of the pre-season games. Jack Hannath has been around the mark and is one of the players we drafted for this exact reason. He's a big body and runs well."
West Coast also have a decision to make in the ruck department after Nic Naitanui failed in his bid to be fit for the season opener.
If the Eagles decide Cox needs back-up in the ruck, either Scott Lycett or Callum Sinclair will be called into the side.
West Coast will also be without midfielder Daniel Kerr (knee), forward Mark Nicoski (hamstring), wingman Matt Rosa (quad) and former `Pie Sharrod Wellingham (ankle).
