The prospect of a clash between the losers of the two preliminary finals was raised at a meeting of NRL club bosses last week, and has head office support.
It's proposed the match would form part of an expanded NRL grand final week, with an Indigenous All Stars clash and under-age State of Origin clash.
The NRL is open to introducing the grand final extravaganza as early as this year, with the possibility of prize money to act as an incentive to clubs.
But Mann said no amount of money would make him believe the game was worth playing in.
"I just think the risk of injury and losing players the NRL wants on the field the next year is too high," said Mann, who has played in six finals series.
"You just wouldn't want to risk it.
"(The finals) are a big emotional rollercoaster, and usually when you have a massive emotional high and then you have the big comedown ... the likelihood of getting injured next game is pretty high.
"You don't really want that to happen to any of the players, especially if there doesn't need to be. If there's nothing in it for them, then it probably doesn't need to happen."
The concept is similar to that at the FIFA World Cup, where this year's third-place playoff between England and France was the tournament's highest-scoring match.
Both sides tend to rotate their squads for the matches in World Cups, giving younger players game time while resting older stars.
Bulldogs boss Aaron Warburton is understood to have championed the idea at last week's NRL meeting.
The Rugby League Players Association had not received full details around the idea as of Monday afternoon, but are known to prefer a two-week build up to the grand final.
Canterbury prop Max King also indicated on Monday he would likely not be in support of the concept, after playing in the past two finals series at the Bulldogs.
"I'll do whatever I'm told. I'm a football player, it's a business, so I'm respectful enough to know that," King said.
"But also it is one of those ones - it's such a tough sport that we play, you leave everything out there.
"You go home after a game and the next two or three days you're just a wreck. Particularly with semi-finals intensity, you're putting everything out there.
"So it would be tough knowing you're out, but then getting up for a third-place playoff ... But I get paid to play, so whatever they tell me to play I've got to play it."
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