Frank Grillo Stunned Costar John Cena Got Hair Transplant, Had No Idea!

It's safe to say John Cena didn't give any of his "Peacemaker" costars a heads up about his hair transplant ... 'cause TMZ broke the news to Frank Grillo this week -- and he couldn't have been more floored by the reveal.

We chopped it up with Rick Flag Sr. out at LAX ... and while we chatted a bit about the new season of the hit Max show dropping later this year (skip the first 30 seconds of the clip if you haven't watched S1 yet), the topic of the WWE Superstar's recent life-changing decision made Grillo stop in his tracks.

In fact, the "Purge" franchise star didn't fully believe our camera guy was spitting facts ... so he said he was gonna reserve comment until he knew for sure we weren't pulling a month-delayed April Fool's prank on him.

Cena made it clear after WrestleMania it did, in fact, happen ... blaming bullying wrestling fans for forcing him to address his bald spot.

One thing Grillo was OK talking about was the fact his lettuce is homegrown ... so we take it Cena will deal with that next time they cross paths.

And if they do end up fighting, we just so happened to ask Grillo how that would go -- check out the clip.