PORT Adelaide will likely turn to third-year defenders Josh Sinn or Dylan Williams to cover the season-ending injury to backman Kane Farrell in the finals.

Fellow defender Lachie Jones expects eight-gamer Sinn and 27-gamer Williams to be at the front of the selection queue to replace Farrell, who will soon have surgery on a torn hamstring.

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Farrell's injury is compounded by the five-game ban to All-Australian half-back Dan Houston, who will also miss the entire finals campaign.

"It's a big hit for us, obviously Dan two weeks before and 'Faz' this week is probably two of our most attacking defenders gone," Jones told reporters on Wednesday.

"It's not great with the two boys going down, down back ... but I have got confidence that whoever plays on Thursday night will have what it takes to stand up against a pretty strong forward line."

The Power host Geelong in a qualifying final at Adelaide Oval on September 5.

"Sinn and Williams are probably the two knocking on the door for the last few weeks," Jones said.

"They're quite alike as players, obviously different levels of experience, but I think those boys will be able to fill a role.

"The whole backline as a group will stand up and do what needs to be done."

Jones has signed a two-year contract extension to remain at Port alongside utility Jeremy Finlayson, who has inked a one-year deal to continue at Alberton next season.

Finlayson (lacerated spleen) is also sidelined for the season.

Port downed the Cats by six points in round nine, the first win in Geelong by the South Australian club since 2007.

And Jones expects the Power to follow the blueprint of that victory in the qualifying final.

"We know they've got some big threats, (Jeremy) Cameron probably being their biggest, just given the license to do whatever he likes," Jones said.

"I think (Miles) Bergman went to him last time and did a pretty good job; Jed McEntee went to (Tom) Stewart and nullified his impact.

"We've got a pretty good gameplan from last time we played them ... we've beaten them earlier in the year in a place where we haven't for a while.

"And we take great confidence out of the fact that we have done that and we have got the players that can stand up and do big jobs."