The Pilgrims dropped out of the top seven following Monday's 1-1 draw with Gloucester City.
Victory at The Citadel would still ensure United a return to the play-offs, as long as they score two more than Kettering Town, who travel to Curzon Ashton.
However, with United, the Poppies, Alfreton and Spennymoor all vying for seventh on the final day of the season, even a defeat could be enough for Cox's side.
"We've got it all to do now and go again on what's a huge day, a cup final for us," he said.
"I've said there might be some twists and turns, it might go to to the 93rd minute on Saturday.
"What we've got to do is get the boys refocussed and make sure we approach the game right and give it our all.
"We've got to go do our part and see it out, bee positive, try to score goals and try to win the game."
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Saturday's contest is a big one for Farsley themselves who need a point to guarantee their survival.
However, Cox admits that's not his concern.
"Farsley are fighting for their lives and we've got to be selfish," he added.
"We've got to try to win a football match."
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