New Zealand international Williams and possible Kangaroo bolter Tupou have been the shock weapons during the finals series for the Bulldogs and Sydney Roosters respectively.
They shape as the super subs in Sunday's decider at Telstra Stadium following what's expected to be a torrid opening quarter.
There's no doubting their talent and ability, but Fitzgibbon says other reserves can also pack a mighty punch.
"I think it's the underrated guys who are slipping under the radar," said Fitzgibbon.
"I know Corey Hughes probably changed the game for the Bulldogs the other night and the same with Reni Maitua's try (in the preliminary final win over Penrith).
"Sonny Bill had a blinder and Roy Asotasi carried the ball well.
"The four bench players were the ones who turned the game and yet for us, we've got very underrated players who come on.
"We certainly lose nothing when we make an interchange.
"Those players don't get the raps they deserve, particularly blokes like Chad (Robinson) and Neddy (Catic)."
The 19-year-old Williams has already played test football, debuting against Australia in April, and is preparing for his maiden grand final after just 14 first grade games.
His stinging defence, superb off-loads, durability and size have the critics raving.
"He's definitely a superstar in the making," said Great Britain and Sydney prop Adrian Morley.
"He's got all the skills and you forget how young he is, the things he does on the park.
"It's a scary thought but he'll keep getting better."
Morley's teammate Luke Ricketson rated the 21-year-old Tupou on an equal footing with Williams, who missed three months of this season with an ankle injury.
"He's a different type of player," Ricketson said about Tupou, who broke his leg last year when set to make his first grade debut.
"They talk a lot about Sonny Bill's make-up, personality and the way he runs onto the field.
"I think Toops has got exactly the same deal. He's a confident young kid who's earmarked to be a great player."
Roosters skipper Brad Fittler added: "I've seen what he's gone through to get where he is today.
"He's a bloke who never lets you down. He works his arse off.
"He'd be right up there (with Williams). He's a good hitter, he can break a game with a try."