Mannah played in the last Eels outfit to reach the grand final - the 2009 side that advanced from eighth place before losing to Melbourne, a title later stripped from the Storm for salary cap breaches.
Parramatta host Canberra in a knock-out semi-final on Friday night, with the winner playing North Queensland away the following Friday for a grand final berth.
The Eels are playing their fifth finals series in six years. At the end of 2022 they farewell back-rower Isaiah Papali'i and hooker Reed Mahoney, who Mannah said were among the competition's best players in their positions.
Mannah said the Eels' premiership "window" was ajar and that they must go through it while it remained open.
"You talk about having a window. Right now is the most crucial time for Parramatta in terms of delivering because I don't know when they will get another opportunity as good as this," Mannah told AAP.
"No disrespect to Canberra and the Cowboys, but if you told Parra at the start of the year they would have to beat them to make the grand final, they would have taken that.
"In 2009 we played with confidence. It is scary, but it was similar to what Canberra is doing where they've gone on a run. We have to be on, but if we play our best we should win."
In 2009, the Eels pack was powered up front by New Zealand internationals Nathan Cayless and Fuifui Moimoi, and included Kangaroos forward Nathan Hindmarsh.
"You could argue this pack is better than 2009," Mannah said.
"We are against two of the best front-rowers (Josh Papali'i and Joseph Tapine) in the competition but Reagan Campbell-Gillard and Junior Paulo are in that class. (Second-rower) Shaun Lane is in career-best form and Ryan Matterson at lock is very handy."
Mannah, who assists a variety of cancer charities after losing his brother Johnny to the disease in 2013, was on Monday assisting sick children at Ronald McDonald House in Sydney by providing them with a medical garment called a Supertee.
The Supertee was created by Jason Sotiris, a tradesman whose daughter was diagnosed with cancer as a one-year-old, and who noticed her tubes and drips would become tangled in her clothes.
"What Jason is doing is giving parents the opportunity to brighten their child's day," Mannah said.
"For a kid in hospital who is always covered in tubes, to be able to wear a Supertee makes a massive difference.
"Losing Johnny was an eye-opener and made me realise how many people have sick siblings, children and parents. Seeing a loved one struggle is a process you don't wish on anyone, especially children."
