Paul Heyman swears he had no idea Brock Lesnar was gonna retire at WrestleMania ... but he tells TMZ Sports the future of WWE is in good hands nonetheless ... 'cause he's now focused on The Vision!!

For those who don't remember, the Hall of Famer told us The Beast Incarnate had no plans to retire prior to last month's festivities in Las Vegas ... and went as far as to say Lesnar still had another 15 years in him.

Fast forward a week and a half, and his guy took off his gloves and boots and called it a career after falling to Oba Femi ... and Heyman was a puddle of emotions in the ring as they hugged it out.

So, when we ran into him in NYC this week ... we had to ask if he was pulling a quick one on us -- or if he was legit stunned with Lesnar's decision.

"If I knew he was gonna retire, would I have gone on record saying he was gonna dominate for the next 15 years??" Heyman asked ... while firing a shot at our fearless leader, Harvey Levin, in the process.

"I was crying," Heyman added. "Am I that good of an actor that I can cry on cue?? Not I."

Okay, now that we addressed the "shocking" retirement ... what's next??

"Bron Breakker, Logan Paul, Bronson Reed, Austin Theory -- The Vision taking over WWE."

Ya heard it from the Wiseman himself.