Jason Collins -- Back Off Michael Sam ... 'Dancing' Doesn't Mean He's Over Football

Michael Sam's decision to join "Dancing with the Stars" does NOT mean he's throwing in the towel on his NFL career ... so says his friend Jason Collins who says he can't wait to watch the guy kill it in the ballroom.

Sam's been taking heat from critics who say the decision to join "DWTS" shows he's more concerned about being a celebrity than playing in the NFL.

But Collins -- who famously came out as the NBA's first openly gay player in 2013 -- says the haters are wrong ... and he's got proof to back up his argument.