Clarke injured the posterior cruciate in his right knee at the Falls Music Festival on New Year's Eve.
On Monday the 23-year-old was confined to Fremantle Oval's weights room wearing a heavy knee brace as the Fremantle Dockers trained outside.
Jonathon Griffin and Kepler Bradley are both returning from knee reconstructions and could be ready to return ahead of Clarke who will be sidelined for up to eight weeks.
Sandilands was uncertain who would take the role as his back-up when all were fit.
"I don't think there's a preferred second ruck option," Sandilands said.
"I think whoever the coaches pick can always go out there. You're happy to play with whoever comes into the side."
Clarke grasped his opportunity with both hands last season following the injuries to Griffin and Bradley. He played 17 matches, including all three finals, in his best year at senior level.
Ironically after getting his opportunity through injury he may have to fight to get his spot back if he is not fit for round one.
"Zac's a really fit guy and it was an unfortunate thing that happened," Sandilands said.
"He knows he's got to do the work now to get back up and running. But he's the sort of guy that runs pretty well and is pretty fit so he won't lose too much at all."
Griffin and Bradley have been running since September after the pair ruptured their ACL's in May. Both men ran solidly in the rehab group at training on Monday and Bradley participated in several drills with the main group.
"Griff and Kep have come back really well," Sandilands said.
"Both are ticking all the boxes in terms of time frame and when they want to be back out there.
"So it's good to see them back out and running around. Kepler was back in with a few drills today which is good."
The Freo Dockers' first match of the NAB Challenge is on Tuesday 18 February against West Coast before facing the Western Bulldogs six days later at Etihad Stadium.
Sandilands says he is more than happy to play in both pre-season fixtures.
"I think it would be important to get the match fitness so I'd be keen to be playing in both," Sandilands said.
"To be honest, I haven't really spoken to Ross (Lyon) about it but it's always pretty important to get a couple of good games in under your belt before round one."
