"I told him if he comes inside, I'll just lean into it," Buck joked. "I told him he doesn't throw hard enough."
Such logic may result in nothing more than a sizable bruise -- hardly thanks for the years Buck spent behind the plate in Kansas City.
But Buck has already moved on. Now, at age 29, after being cut by the Royals to make room for Jason Kendall, Buck and Greinke have become baseball enemies. Greinke must continue to pitch in Kansas City. And Buck must learn a new staff and develop a new ace or two in Toronto.
The success of the Blue Jays may depend upon it.
"It's a fresh start," Buck said. "We've obviously got a young staff, like the way Kansas City was when we were really young. But the guys that are young that are here, I think they're ready to be at this level."
Buck, already, should know. Upon signing with the Jays in December, he contacted the team to request film on every member of the Toronto staff. Running the tape constantly while he played with his twin boys, Brody and Cooper, in the office at his home, Buck managed to steal a glance every now and then at Shaun Marcum's pitch selections and Ricky Romero's arm slot.
For a newcomer, Buck already knows plenty about Toronto's pitchers. Brody and Cooper aren't far behind.
"It's a fresh start. We've obviously got a young staff, like the way Kansas City was when we were really young. But the guys that are young that are here, I think they're ready to be at this level."
-- John Buck
"If I need help, I can always check with them," Buck joked.
Chances are he won't need help. Every now and then, when his sons dozed off for a midday nap, Buck delved deeper into the video, aiming to gain an encyclopedic knowledge of every last Jay. By doing his homework, Buck figured, he could more seamlessly transition into the team's new everyday catcher.
Studying everything from the pitchers' arsenals to their tendencies, Buck worked to make the staff his own.
"It helps them be more comfortable," he said. "I didn't want to come in and say, 'This is what you're going to do.' They're good for a reason. They're here for a reason. We can use that and go from that point, rather than me coming in and not knowing them. I think that helps them just stay with who they are."
That's not to say, however, that Buck will shy away from tutoring. Quite the opposite.
Take Sunday, for example. After catching Dustin McGowan's first session against live hitters in nearly two years, shouting sporadic words of encouragement, Buck stuck around the batting cage to catch some more live batting practice for Brett Cecil. As soon as the lefty finished, Buck pulled him aside for some pointers.
"That's one reason we got Buck," Jays manager Cito Gaston said. "We heard he's great with young pitchers."
Greinke was one of them. Cecil, Romero and the other young Jays could be next.
If Buck also hits, as he did to some extent in Kansas City, the Jays will consider it a bonus. Last season with the Royals, limited to 186 at-bats due to a herniated disk in his lower back and a split of the catching duties with Miguel Olivo, Buck still managed to whack eight home runs and slug .484. Though he won't hit for average -- his .247 mark last season was a career high -- Buck has shown consistent power throughout his career.
The Jays will take just about anything offensively from a player known for his ability to handle a pitching staff. He proved it in Kansas City, doing his part to help mold Greinke into one of the game's top pitchers. He will attempt to prove it again in Toronto.
And if he manages to reach base against his old batterymate along the way, all the better.
"Remember, I caught him, so I know how good he is," Buck said of Greinke.
But his plan is in place.
"Maybe he'll hit me," Buck joked. "It's not like I haven't been hit by one of his pitches before."