Fonua-Blake starred in the Warriors' 24-12 win over Canterbury, breaking through for his fourth try in five matches and running the most metres of any front-rower.
Payne Haas is widely regarded as the NRL's best prop, particularly given his ability to play 80 minutes with sheer power for Brisbane.
But Fonua-Blake has entered the conversation for the game's next-best middle.
He is second only to Haas for tackle busts this year, while also in the best season of his career by averaging 148 metres per game.
"I certainly think he's got the ability to be the best," Harris said.
"What I liked about Addin is he hasn't got that ego about him. He's been doing the little things really well. Probably better than I've ever seen him do it.
"Doing the tough stuff and we've needed that. There's been times where we've had the first 20 minutes of games without the ball. He's just put his head down and done his job.
"Obviously you want plenty of ball for your front rowers to make their mark on the game. But he's just doing it in different ways."
Fonua-Blake is not eligible for State of Origin after representing Junior Kiwis as a teenager, but would otherwise be one of the first-choice picks for NSW.
Warriors coach Andrew Webster is also adamant the 27-year-old is still far from his best.
"He's a role model for the guys. So when Addin is leading well and he plays the way he is doing, it has a big impact on the place," Webster said.
"He's realising his potential.
"He's having a big impact and he just he's a good player. He's got more in him, believe it or not.
"Any club would love a front rower like Addin. We're really grateful we've got him and he's on our side."
Fonua-Blake is one of several Warriors enjoying career years.
Shaun Johnson has had a resurgence as a 32-year-old halfback, while Dylan Walker and Marcelo Montoya are also playing superbly.
Wayde Egan has also impressed out of dummy-half, and while he hurt his calf in Friday night's win he is expected to be fit after the round-12 bye.
Webster said it was no coincidence players had found form on the Warriors' return to New Zealand this year after three COVID-impacted seasons in Australia.
"Stability is everything for everyone," Webster said.
"If you know what the future looks like, then you can really dial into what's happening now. If you don't know that, then you're erratic."
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