Garciaparra batted .455 in 13 games after signing a free-agent deal with Oakland about three weeks into Spring Training.
"Nomar had a great spring for us," Geren said. "He's an outstanding hitter. I'd like to get him in there as much as possible."
Buck, who batted .300 with four home runs and a team-high 16 RBIs in 27 games during Spring Training, took the news in stride.
"Of course you want to start on Opening Day," he said. "But who's going to complain about getting bumped by Nomar Garciaparra? The guy's going to be in the Hall of Fame, and obviously he's still a great hitter."
That Buck is a left-handed hitter, Garciaparra bats from the right side, and the Halos were starting southpaw Joe Saunders, Geren said, had nothing to do with the move.
"He'll play against most left-handers ... [but] he's not a big left-right guy," Geren said of Garciaparra, whose .314 career batting average is the seventh-best among active players with more than 3,000 plate appearances.
He was last year. Garciaparra, limited by injuries to 55 games with the Dodgers, batted .224 (24-for-107) against righties and .339 (19-for-56) against lefties.
Over his career, however, the numbers are nearly identical; entering his 14th big league season, Garciaparra has a .313 batting average against lefties, .314 against righties.
"He's just a solid hitter," Geren said.
Garciaparra, signed to play mostly at third and first base but expected to fill in at shortstop from time to time and get some starts at DH, entered the game batting .241 (39-for-162) at Angel Stadium. It's his lowest career mark at any ballpark in which he's logged at least 60 at-bats.