Coach Kristian Woolf said the star centre would miss Thursday's clash against Canberra at GIO Stadium, where the Dolphins are looking to win a fourth straight game and climb up from eighth spot.
Already missing a plethora of stars because of State of Origin, including fullback Hamiso Tabuai-Fidow, winger Selwyn Cobbo, prop Thomas Flegler and lock Max Plath, the news will only make the task harder for Woolf's side.
Five-eighth Kodi Nikorima sustained a hamstring injury in the Dolphins' 32-10 victory over South Sydney last Friday and will be replaced by former Raider Brad Schneider, while Trai Fuller, Tevita Naufahu, Francis Molo and Jeremy Marshall-King enter the starting 13.
"He (Farnworth) has been a little bit tight through the hamstring through the week, so we modified him on Monday and thought that he would come through," Woolf said.
"He's obviously a key player for us ... but if we throw him out there, and we're missing him for six weeks or eight weeks or something like that, as we did at the end of last year, that's going to hurt more than one game."
Canberra are also without star playmaker Ethan Strange and the barnstorming Hudson Young, who are on Origin duty, but welcome back second-rower Zac Hosking (concussion) and winger Xavier Savage (ankle) for Thursday night.
Regardless, coach Ricky Stuart is excited for 21-year-old Strange, saying he deserves the call-up after dazzling for the Raiders this season.
"I always say that having any young player - or any player for that matter - playing representative football always comes back a different player," Stuart said.
"Hudson always returned to his club football hungrier and a better player. When you're training and playing, preparing with world-class players, it just rubs off on you, and Ethan will be no different."
Recruit Daine Laurie will replace Strange, while Ata Mariota steps in for Young as Canberra look to bounce back from a 30-18 loss to Penrith on May 10.
But the fixture will mean more for second-rower Jordan Uta, who will make his competition debut from the bench.
"I brought him in and chatted to him three weeks ago, and ... I just said, 'Mate, you don't know how close you are'," Stuart said.
"I think he thought he was a lot further away than what I was telling him, and then it was so nice to be able to tell him that you basically got your opportunity down."
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