RUCK recruit Mason Cox could give Fremantle another forward threat in Sunday's Western Derby 62 if the Dockers choose to rotate the premiership-winning big man in attack during his club debut, according to coach Justin Longmuir. 

Cox will play in Sunday's clash against West Coast after being named in the extended squad on Thursday, with Longmuir also confirming that forward Michael Frederick will play his first game this season after recovering from an ankle injury.  

Cox has starred at WAFL level for Peel Thunder and was told by Longmuir to make sure he was in form and ready for an opportunity, with a concussion setback for Sean Darcy opening the door this week. 

While Cox will likely fill the same second ruck role that Darcy has played this season on managed game time, Longmuir said the 35-year-old would also give the Dockers flexibility with his ability to push forward as a marking target. 

"Mason's been in really good form. He's been our best player at Peel … and really he's been just waiting for an opportunity, and an opportunity has presented, so he'll come in," Longmuir said on Friday.  

"My message to him the last three to four weeks was just make sure when the opportunity comes up, you're in good form, and he's done that.

"We want him to be himself and we want him to bring all of himself, and he's done that since he got to the club. 

"He's done it at training sessions, in the gym, and he's been able to be himself. So I wouldn't ask him to not do that on game day."

Longmuir said Cox's game time would be "similar to Sean", who has played between 43 and 54 per cent game time as the Dockers utilise the fifth bench spot for their second ruckman.  

Asked what different strengths Cox would bring to the team, Longmuir said: "The obvious one is his ability to go forward. Whether we use him there much with our three talls, he does give us that flexibility. Otherwise, it's pretty similar (to Darcy)." 

Longmuir said Darcy was recovering well from his concussion and "back to his normal self" after a head knock against Collingwood last Friday night forced him out of the game during the second quarter. 

The Dockers will be boosted for Sunday's Derby by the return of star midfielder Hayden Young, who has recovered well from a hamstring injury and proved his fitness during Wednesday's training session. 

Young has not featured since the round one loss to Geelong after playing through hamstring awareness, with the Dockers giving him an extra week in rehab to make sure he was primed to return.  

Longmuir said the Dockers would continue to use the gun ball-user, who won the Glendinning-Allan Medal as best afield in the most recent Derby, in a midfield-forward role. 

"In the end, it's where he feels most comfortable as a player and where he has the most impact for the team," Longmuir said.  

"So, although his minutes might slightly change first week back in terms of forward/mid balance, he'll continue to play the same role.

"He has too much impact for us in the midfield but also forward of the ball. So we want him in there. 

"He's one of our best centre-bounce players and we want him in a position where he can best impact the game.

"I feel like we're on top of his injuries. I feel like he's had a really good rehab stint and he's put a lot of work into that, so we feel like he'll be all right."

Young made a stunning return from injury late last season, playing a match-winning role as the substitute against Collingwood in round 19 before winning the Glendinning-Allan Medal as best afield in Derby 61, despite being substituted out of the game.   

Asked if Young was ready to perform at his best straight away, Longmuir said the Dockers would be focused on getting the 25-year-old back into the team and playing his role. 

"We know he uses the ball well, and we know he has real strength around the contest," Longmuir said. 

"He was probably right to go last week if we were really in a rush, but we wanted to get him right.

"He's trained really well and we're expecting him to come in and play his role."