Cowboys coach Todd Payten had named co-captain Dearden on an extended bench earlier in the week as he eyes a return from a syndesmosis and ankle fracture that has kept him out of action since round 10.
"Tom is not fit to play this weekend," Payten said.Â
"He's probably two full training sessions short of being right to play.
"He'll get a full week of training next week, with the plan to play against the Dolphins (in round 20)."
Purdue, who can also play anywhere in the outside backs with distinction, will get another shot at No.6 against the Sea Eagles on Sunday.
The 20-year-old did a stellar job in the rousing 26-12 win over competition leaders Penrith in his previous match where he showcased his creativity, running game and ability to defend in the front line.
Purdue had two line-break assists, made 10 runs for 80m and more importantly completed 27 tackles with none missed. The Panthers ball runners singled him out, but the 83kg Purdue answered every challenge.
"It has been fun. Compared to last year I can read the game better," Purdue said of his five-eighth switch from centre.
"I'd forgotten the fitness side of being in the halves but I've played there all my juniors. I just had to find my feet and play a bit more free which is what I've been doing.
"Defensively I wasn't where I need to be but the last few weeks I've felt comfortable in my defence.
"To get a win against a quality Penrith side really gives us confidence."
The clash at Brookvale Oval will have major ramifications for the Cowboys' finals hopes.
Manly are in seventh position on 22 points. The Cowboys have the same points tally but are ninth due to their poorer for and against.
While the Sea Eagles have won eight of their past 10 matches against the visitors, the Cowboys are the only NRL team to have a positive winning record against Manly at Brookvale Oval with seven wins from 13 games at the venue.
Their last trip in 2025 wasn't a happy one with Manly winning 42-12.
"Last time I was there I got pumped," Purdue grinned.
"You have to go to Brookvale with a lot of energy and motivation to get the job done so hopefully we can get points early and quieten the crowd. We've got to throw the first punch."
Queensland trio Reuben Cotter, Jeremiah Nanai and Murray Taulagi are all set to play in the match and back up from State of Origin.
Taulagi, named on the wing, was on a six-man bench for the Maroons but did not get on the field on Wednesday night so will be fresh.
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