Rosado -- a former WBO light middleweight champ -- was out in L.A. when he told us Michael was like a sponge during the production ... soaking up enough boxing knowledge to make him a real life ass kicker.
"Michael can throw down man. If you sleep on him in the street he might put you to sleep."
Gabriel would know a real fighter when he sees one ... the dude's been in the squared circle with champions like Gennady Golovkin and Peter Quillin.
Bonus awesomeness ... you get to hear Rosado talk about having to remind himself that Rocky Balboa wasn't a REAL champion fighter when he met Sly Stallone for the first time. It's pretty funny.
R.I.P. Apollo Creed.