FREMANTLE star Josh Treacy and No.1 ruckman Sean Darcy are both set to miss Saturday's important clash against Greater Western Sydney after the Dockers confirmed mature-age ruckman Liam Reidy would make his debut. 

Key forward Pat Voss also joined teammates ahead of their flight east on Thursday as the Dockers prepare to cover for the important pair, who remain sidelined with knee injuries for a season-defining clash. 

Reidy will earn his chance in his second season on the Dockers' list after being recruited as a mature-age rookie from Frankston. 

He has produced excellent WAFL form but was unable to replace Darcy last week as a late inclusion after being left off the club's emergencies list in favour of experienced key forward Matt Taberner. 

With Luke Jackson viewed as more suited to the Geelong ruckman last week, coach Justin Longmuir said it was important for Reidy to instead play WAFL minutes last week in case Darcy remained unavailable against GWS. 

Darcy did very little training at Wednesday's main session while Treacy was not sighted on the track as he recovers from an impact knee injury against Essendon in round 21. 

"This is why you have a list of players, and players have been waiting for opportunities all year, and a couple have presented themselves in the last few weeks," Longmuir said on Wednesday. 

"We get to test that depth and test our squad, which is what you have a squad for."

The 24-year-old Reidy's inclusion will allow Jackson to remain forward and steady a forward line that will not have Treacy, who has emerged as a star this season with 45 goals.

Voss worked hard to create several opportunities to impact the last-gasp loss to Geelong but struggled to hold his marks in his fifth game for the club since joining as a pre-season recruit. 

A strong forward, he has shown the ability to challenge backlines in general play but lacked the polish to finish his work with 3.10 since debuting for the club in round seven.