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Cam Newton's days as an NFL superstar are NOT over ... so says his Panthers teammate, Greg Olsen, who tells TMZ Sports he's got all the confidence in the world Cam will be back.

"Don't bet against Cam," Olsen says.

Newton has had a rough last few seasons battling injuries and poor play ... and the Panthers have actually been BETTER without him this season, going 3-0 in the games that he's missed.

So, when we saw Greg out in NYC this week ... we had to ask, is it over for the Carolina QB???

That's when Olsen told us no way, saying, "Don't bet against Cam. Once he gets himself right, he'll be ready to go."

As for the foot injury that's sidelined Newton for the past month ... Olsen says from everything he's seen, Cam is progressing just fine.

"I think he's feeling good," Olsen says. "I think he's feeling better."