Reggie Bush tells TMZ Sports he's training his ass off to get back on an NFL roster -- but if he doesn't get signed in 2017 ... he'll retire. 

Bush is only 32 years old and he's still in tremendous shape ... but his last 3 seasons haven't exactly been productive -- and now he says he's waiting for a call. 

The running back says he's never been in this situation before -- but has already made his mind up ... if he doesn't play this season, he'll move on with his life. 

By the way, Bush -- who was a pretty solid pass catcher -- recently said he would switch positions to WR if asked. 

Belichick ... you listening?