Scary revelation from Rob Gronkowski ... who says he's probably had 20 concussions in his life from playing football including 5 blackout moments -- but he feels like he's "fixed" now.
The New England Patriots legend was talking football to "CBS News" when he was asked if he'd ever let his child (if and when he has children) play the sport he loves.
“I would let my son play football, but first off I would educate him on the game," Gronk said ... "Educate him on what I went through."
But here's the scary part .