Evander Holyfield ain't giving up on Andy Ruiz -- telling TMZ Sports he strongly believes the former champ can win his belts back ... if he busts his ass!
Remember, Holyfield and Ruiz go WAYYYY back -- they famously sparred back in the day when Evander was training for his comeback in 2011.
Ruiz was a nobody way back then ... but Evander has said he could tell Andy had killer potential and wasn't shocked when he beat Anthony Joshua back in June.
Of course, Ruiz lost the Dec. 7 rematch -- and blamed himself for eating and partying way too much -- but Holyfield says if Ruiz can get focused again, he's a legitimate contender.
"Joshua became champion again, right? He made an adjustment," Holyfield tells TMZ Sports.
"People who make adjustments, their chances is still good. He made some bad choices. When you make bad choices, you gotta be accountable. If you're accountable, you can make adjustments."
Holyfield -- who knows all about coming back after losing a belt -- also had some advice for Ruiz.
"Go back to the work plan. Do whatcha daddy tell you to do. All these years, he's been right, what makes you think he's goin' be wrong?"
There's more ... boxing superstar Terence Crawford also believes in Andy ... telling TMZ Sports he understands why Ruiz lost his focus but strongly believes he can bounce back.
In fact, Crawford says he's good friends with Ruiz and knows what he's capable of doing in the ring when he's not raging outside of it.