TMZ Sports talked to 37-year-old Trout, the former WBA light middleweight title holder, ahead of his February 17 bare-knuckle fight with 41-year-old Sanchez ... and asked him if this was something he planned to do more than once.

"Don't get me wrong, I'm still very much trying to pursue a world title in the boxing world. But, there's no reason I can't be a champion in both. I hope it can be a lasting relationship because I like what they're doing in the BKFC," Austin told us.

Trout has a professional boxing record of 36-5-1 ... while fighting some of the best fighters in the world, including Canelo Alvarez, who Austin took on the full 12 rounds with back in 2013.

Of course, Diego is a UFC Hall of Famer who beat everyone from Nick Diaz to Clay Guida ... over his illustrious career.

Naturally, you'd think Trout, a world-class boxer, would have the advantage over a guy whose best weapon was wrestling -- and that may be true -- but it's not how Trout is approaching the fight.

"I'm not gonna think that way. I have to make sure I prepare for the best Diego can give me, and if he makes it an ugly fight, real rough and tough, it's gonna be a long night for me," Austin said.

"So, not only do I have to be able to neutralize that, but I also need to be able to whoop his ass if he decides to come inside and square up with me. We gotta make him pay in every aspect."

We also talked to Diego and asked him how he planned to hang with an elite boxer.

"BKFC is not boxing. For all those people out there hating on me saying, 'Oh, what's BKFC thinking? Diego in there with Austin Trout?'"

"C'mon guys, boxing is different. MMA is different. BKFC is definitely different. Austin has been picking his own opponents for the past couple of years. I was fighting nothing but beasts at the end of my UFC career."

Despite the strong words, it's clear Diego and Austin, who have known each other for years, have absolutely no bad blood between them.