Duffy will make his Broncos club debut in the NRL at halfback against his former side the Cowboys at Suncorp Stadium.
Reynolds was away from the main group at captain's run on Thursday, sprinting and running side-to-side to test his injured groin, but his valiant attempt to play was a bridge too far.
"Reyno did all the running. He was okay with that, but he just felt the kicking part was something that wasn't where it needed to be," Broncos coach Michael Maguire said.
"So we're fortunate to have Tommy ready to go."
The main squad trained with Duffy calling the shots at halfback alongside No.6 Ezra Mam.
The 22-year-old, who will also kick goals in the absence of the skipper, has been a sponge around Reynolds and fellow injured playmaker Ben Hunt.
"I'm really looking forward to seeing Tommy get his opportunity. He's fitted in really well. He came in on day one, ripped into the pre-season and he's earning everything he's doing," Maguire said.
"He's really good at being able to organise a team and he's got a great kicking game.
"Tommy, he just needs to play his part and do his job, as we say. I know he'll do that for us."
Former Broncos half Jock Madden has slotted in for Jarome Luai at Wests Tigers and done a terrific job by playing to his own strengths, and Duffy just needs to do the same.
"We want Tommy to play Tommy style. Under the intensity of the game I'm looking forward to seeing him push the boys around and play his style," Maguire said.
The Broncos will elevate playmaker Josh Rogers to the six-man bench to provide cover for Duffy.
Versatile outside back Jesse Arthars will play fullback in place of injured superstar Reece Walsh and the coach has ultimate faith in him.
"He's just selfless and a great man, Jess. He just fits in wherever. He'll go out and work his backside off for his teammates," Maguire said.
The Broncos will miss Walsh but the coach is backing a genuine team effort to assist Arthars.
"I think if everyone's doing their role, that allows Reece to do what he can do. And it's no different with Jesse out the back," Maguire said.
The Cowboys , like the Broncos, have won three games in a row and always lift for this Queensland derby fixture.
"They're always great games," Maguire said.
"We're pretty determined to keep building on what we're doing but we're not happy with everything we're doing. We're chasing different parts of what we need to be better at."
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