Yeah, the trash talking has begun between the ex-MLB superstar and his celebrity boxing opponent ... and Lenny's already aiming below the belt.

"On the 28th of September, you're gonna see a f**king ass kicking," Dykstra told us at the Vivid Cabaret in New York City ... "If I don't beat him, I'll be a ballerina dancer."

What's interesting about this clip ... Lenny says he wants a SECOND fight after Bagel Boss -- and he's calling out his longtime enemy, Mickey Rourke.

Remember, Lenny and Mickey have been BEEFING for years -- ever since Lenny went on Howard Stern back in 2016 and accused Mickey of stiffing him out of a $30,000, suggesting the debt was drug related.

Back then, Mickey threatened physical violence against Dykstra ... telling TMZ Sports, "I would knock the living sh*t out of him the minute that I see him."

Well, Mickey might get his wish ... because Lenny says once he obliterates Bagel Boss Guy, he wants to fight Rourke in a celebrity MMA fight.

"Remember Mickey, I know you," Lenny said ... "You're a f*cking p***y."