Fremantle are flying at the top of the AFL ladder, but there is still a big headache for Justin Longmuir ahead of their clash with Port Adelaide.

The Dockers sit two games clear of Sydney in second place after their comeback win over the Swans in a pumping Thursday night encounter.

A key takeaway from the night was the performance of second ruck Mason Cox, who was brought back into the side after Sean Darcy was managed.

The premiership Pie grabbed his chance with both hands, taking six marks and kicking a goal in his best performance in purple.

It left Longmuir to admit Darcy was no guarantee to come straight back into the side.

“It’s tight; we’ll have to wait and see. We’ve got another training session to go, and the team doesn’t need to be in until Thursday so we’ll make a decision on that later in the week,” Longmuir told 7News.

“They both bring different things to the team; Sean’s ruck work when he’s up and about is outstanding, and Coxy gives us flexibility, not only going in the ruck but going and having an impact down forward, which we saw on the weekend.

“It was probably one of his better games for the club; his aerial was strong, and I really liked his second efforts, so it’s a discussion for Thursday.”

It’s not the only selection headache facing the Dockers, with star midfielder Hayden Young set to return from a groin injury.

“He’s got to tick a final box of getting through main training on Thursday, but at this stage it’s looking really positive, and he trained really strongly today so he’s in a good space,” Longmuir said.

In contrast, Port Adelaide will be forced to dig deep into their reserves after Zak Butters and Mitch Georgiades were both injured while Jason Horne-Francis was banned in the loss to St Kilda.

It could see former Docker Will Brodie earn an AFL recall, and Longmuir said they would not take the Power lightly.

“They will have a different look side this week, and we spoke about that with the playing group, but their game is in good order. Carry [Josh Carr] has done a great job there in his first year,” he said.

“Defensively, their profile is outstanding; they defend the ground and the back half really well, and their contest is in good order, so just because they’re missing some of their players, we aren’t going to take them lightly.

“Their game is in good order, and it will take their best.”