Evander Holyfield actually traded punches with a young Andy Ruiz back in 2011 ... when Andy was only 19 ... and the "Real Deal" tells TMZ Sports he knew back then the guy was a stud.
"He's good, we sparred when I was 46 and he was 19," Holyfield said.
"He showed that he was good then. I'm sure when people continue to practice, they work, they continue to get better."
The footage of the 2011 sparring session is pretty great -- the two are both in headgear, but they trade some pretty solid shots .