"He just moved a little closer to the plate," Geren said of Buck, who went 3-for-5 with a double and a game-tying, ninth-inning homer off Red Sox closer Jonathan Papelbon on Tuesday. "He has a lot better plate coverage now, and he's not any more susceptible to [pitches on] one side of the plate than the other.
"It was a short window where they were working him outside. Maybe three, four, five games. Now everything's changed."
Buck enjoys the cat-and-mouse game that comes with the advanced scouting methods used in the Majors.
"In the Minors, there's not as much game-planning involved," Buck said. "Certain teams might try to pitch you a certain way, but not the whole league, like it is up here. It's like, once they find a weakness up here, or something they think is a weakness, they go after it. It's up to you to figure that out, and the faster you do it, the faster you get back on track.
"But then they'll try to find something else. It never really ends, and that's kind of cool, that game within the game."
Prior to Tuesday's breakthrough, Buck was mired in a 3-for-33 (.111) slump.
"He's got a good head on his shoulders," Geren said. "He's an impressive guy in just about every aspect, really. It's tough being a rookie, but he's handled himself well and doesn't get too high or too low.
"That's the right approach to have if you plan to stick around a long time."
Buck was in the starting lineup posted before batting practice on Wednesday but was a late scratch with a sore right wrist.
High hopes: First baseman Dan Johnson went 0-for-3 in his first game back from what was once thought to be a season-threatening hip injury. In his next five games through Tuesday, he went 6-for-17 (.353) with five walks, including a clutch night in the series opener here.
He belted his first homer of the year off Curt Schilling, then hit a game-winning double in the top of the 10th inning to bump his average to .300.
"He talked a lot this winter about wanting to use the whole field more, and he's been doing it in batting practice, and he's doing it in the games," Geren said. "He looks great. I think he's going to have a big year. I really do."
Dribblers: Outfielder/first baseman Nick Swisher tested his strained left hamstring with some running on the field before Wednesday's game and remains on track to return to the lineup on Friday for the opener of a three-game series at Tampa Bay. ... Switch-hitting outfielder Milton Bradley took part in early batting practice, hitting exclusively from the right side of the plate. Geren, who was the pitcher for the session, said that Bradley looked great. ... The next time you're inclined to grouse about the $15 parking fee at McAfee Coliseum, think of this: In the neighborhood surrounding Fenway Park, some private lots are getting as much as $50 per car.
Up next: The A's are off on Thursday. Righty Dan Haren (3-2, 1.60 ERA) will get the ball on Friday opposite Devil Rays lefty Casey Fossum (2-2, 8.58 ERA). The first pitch is set for 4:10 p.m. PT.