Okay, sure. It was called, as "usual" (for MLB).
But this time, they didn't. No ball-to-be-played enters the game without going through the PU's ball bags and hands (yeah, yeah... I had to qualify this because of the occasional "free baseball at the ballpark giveaway night!", or the youngster mimicking the adults and "throwing it back"). So if they didn't change out the baseball, then Peralta (F1) never had a/the baseball... certainly, PU Bucknor never issued him (a new) one.
But here's the wrinkle... and again, I've mentioned it before several times here on U-E, it's unique to Major League Baseball. As soon as F1 engages the rubber, the ball (that he's supposed to have) is made Live regardless of PU signaling the ball Live or speaking it ("Play") Live. The action clock begins, and the Official Timekeeper (a League employee) is cued off such an act as the pitcher (re)engaging the rubber.
So the ball is supposed to be Live upon Peralta engaging the rubber... but he doesn't have a/the ball. Thus, a call of Balk is appropriate.
By contrast, in other leagues and rule codes (amateur, specifically), a/the ball is only made Live when the PU signals/says it Live. We ran into this exact problem in a league this summer – after a Foul ball, a new ball was thrown to F1, who received it within the dirt circle of the mound. The clock operator was mistaken, and started the clock. R2 was still returning back to the vicinity of 2B, and the F1 threw to F6, who "tagged R2 Out". The ball was never put back in play – signaled or spoken Live – by the PU. Caused a bit of a ruckus. Because coaches on both sides were wondering:
Why wasn't he (R2) Out?
If he's not Out, then why wasn't that a Balk?
"But the clock started!"
We aren't Major League Baseball.