Here's the deal ... the '90s boy band's hit song "Bye Bye Bye" blasted into the Top 20 on Spotify's global chart this past week more than two decades after its original release -- all because Ryan Reynolds featured the tune in the opening credits of "Deadpool & Wolverine."

Since their names are being thrown around again as a result of the movie -- folks are yet again clamoring for the guys to get together for a tour (or even new music) ... but Chris tells TMZ there's a major hang-up that might prevent that from ever happening.

For starters, there's already been interest in *NSYNC getting together again -- dating back to the 'Trolls 3' movie -- and CK confirms 'Deadpool' is only adding more fuel to that fan fire.

Amid all that fervor ... Chris says there's been rumors -- or perhaps just an assumption -- that he, Lance Bass, JC Chasez and Joey Fatone would do an *NSYNC reunion without Justin Timberlake, who hasn't shown any interest in this, but Chris shoots that notion down.

As he explains to us here ... an extended would-be *NSYNC reunion of any kind is all or nothin' when it comes to its members -- and Chris says they ain't doing jack squat without JT.

His rationale basically boils down to the fact that they're a group -- and breaking up the group just isn't going to happen ... even if he and the other fellas are perhaps more eager to get something going than Justin is. He makes it clear ... no Justin, no *NSYNC.

We know Justin is still friendly with the guys -- he did recruit them for a new song on his most recent flick, after all -- but he hasn't dropped any explicit hints that he's interested in getting the band together officially. Of course, he's on his own world tour right now.

Anyway ... in terms of how 'BBB' came to be used in 'Deadpool' -- Chris gave us the backstory on that too ... and it's pretty neat. He says Ryan approached them and gave them a sense of how it'd be incorporated ... but when they saw the final product, they were wowed.

Of course, because the movie's a smash hit right now ... that means 'Bye Bye Bye' is back on the charts -- but like Chris says in no uncertain terms -- fans can probably cool it on calling for a reunion ... at least until JT gets on board, and it's unclear if he will.