Fremantle ruckman Aaron Sandilands is out for the rest of the season after suffering a serious hamstring injury against St Kilda.

Sandilands left the ground in the first quarter of Sunday's nine-point loss, and didn’t return.

The 34-year-old had just returned due to hamstring tendon inflammation in the same leg.

"It's season-ending, so that's the end of Aaron for the year, but that's no excuse – they lost Gilbert (to concussion)," Coach Ross Lyon said.

"Teams are losing two and three (players in-game). You've got to have the fortitude, desire and mental toughness to get it done.

"I haven't checked-in with (Sandilands). I just gave him a tap on the bum.

"He's a pretty tough character, it's just the environment we're in, stuff happens."

Fremantle will now turn to Sean Darcy, who had 33 hit-outs, 16 disposals and his first career goal against the Saints.

"He clearly competes, doesn't he? He's got some attributes that we like and he's working hard," Lyon said.

"He's got plenty in front of him, plenty of growth."

Fremantle could regain Stephen Hill from a minor calf strain against North on Sunday at Etihad, while Connor Blakely will be available after a club-imposed suspension.