Michael Sam says he's still training his ass off for an NFL comeback ... but knows it ain't gonna be easy -- telling TMZ Sports, "[The NFL's] a hard f**king place."
Sam was leaving The Nice Guy in L.A. when we asked if he had any advice for Johnny Manziel -- since he knows first hand about what it feels like to get cut.
"He doesn't need my advice. He's a grown ass man," Sam said ... "He's a football player. He'll make it."
As for Sam, the 26-year-old says he's laser focused on making a roster next season ... but he knows exactly how tough it's gonna be.