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.
Frank Grillo Stunned Costar John Cena Got Hair Transplant, Had No Idea! | TMZ TV
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..
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