Michael Sam's former college teammate -- who's played with NFL stars like Aldon Smith and Sheldon Richardson -- says Sam is RIGHT ... his sexual orientation is the reason he's not in the league.
We spoke with Marvin Foster -- who played alongside Sam at Missouri -- and says Sam is one of the best players he's ever stepped on the field with.
"There's no way the Defensive Player of the Year in the SEC should be a free agent," Foster said.
TMZ Sports posted footage Wednesday showing Sam very strongly suggesting that he has the talent to play in the NFL -- but isn't on a roster because he's openly gay.
"The kid can play and I think he showed he can play," Foster added ... "He had three sacks in the pre-season. I don't know why they cut him."
But not everyone shares Foster's opinion -- we spoke with an active NFL lineman who tells us, "If Sam was killing he would be on a roster."
The NFL player adds, "He can make an excuse if he wants to ... but that's silly. If you're good, you're good. The NFL teams don't feel that he is."