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Vince McMahon, listen up ... Rick Neuheisel is offering advice to the XFL in the wake of the AAF's colossal collapse -- "Start with enough money!!!"

"At the end of the day, you gotta have enough financing," the former Arizona Hotshots' head coach says.

Of course, the AAF just folded a few weeks ago ... and Neuheisel tells us it pretty much boils down to they just didn't have enough cash to keep it going.

So, when we asked for his message to McMahon's XFL -- which is set to kick off in 2020 -- he told us his advice is simple ... be sure to have enough green to start!!!

As for if Neuheisel thinks the league can succeed once it does that -- the ex-UCLA coach says he thinks there's certainly a place for a pro league in the NFL's offseason.

"I think there's room and need for spring football," Rick says ... "There's a gap between college and NFL and it allows players to show they're capable."