Hulk Hogan -- WWF Rejected Big NFL Stars

Just because you can play in the NFL doesn't mean you're tough enough to WRESTLE -- so says Hulk Hogan who tells us all sorts of NFL tough guys have tried to cross over to the ring ... and FAILED.

And not just no-name scrubs ... we're talking some pretty famous NFLers who couldn't quite hack it with Vince McMahon's organization ... guys like Oakland Raiders legend Lyle Alzado.

Unclear why Alzado -- a maniac on the field -- didn't get a contract with the WWE (then WWF) ... but Hulk says he wasn't the only NFL star who didn't make the cut.

Hogan names names ... Bill Fralic (4-time Pro Bowler), John Matuszak (2-time Super Bowl champ) ... and explains that it takes a special sort of toughness to fake fight in the squared circle.

"You gotta be able to work hurt," Hogan says ... "You can't be taking a day off from wrestling because you twisted your pinkie if you're wrestling the main event at Madison Square Garden."

So, does Hogan think Sam has a shot?? Check out the clip ...