LaRoche tore the ulnar collateral ligament off the base of his right thumb. He was attempting to catch a pickoff throw from Dodgers catcher Danny Ardoin that deflected off St. Louis base runner D'Angelo Jimenez and struck him on his throwing hand. Surgery involves the use of an anchor suture to reconnect the ligament and part of the metacarpal bone that chipped off.
LaRoche was injured in the fifth inning after taking over third-base duties for Nomar Garciaparra, who was injured an inning earlier when he was struck on the right wrist with a pitch from Kyle McClellan. X-rays of Garciaparra's wrist were negative.
LaRoche and Garciaparra were in competition for the starting third-base job, which now goes to Garciaparra, whose disappointing 2007 season left the job open when Spring Training started last month.
Like LaRoche, Garciaparra has been having a strong spring. He is batting .375 and slugged his first home run Friday before leaving the game. It was Garciaparra's loss of power last year that created the opening for LaRoche.
Joe Crede of the White Sox and Brandon Inge of the Tigers are third basemen believed to be available, but Dodgers general manager Ned Colletti said he believes the organization has sufficient infield depth until LaRoche returns.
"Right now, our focus will be internally until Andy's back," said Colletti. "I don't think we have to look elsewhere. Nomar is having his best spring. You don't draw it up like this, losing two third basemen in a matter of outs. Luckily, it looks like Nomar will be out only a couple of days. We have Tony Abreu and Ramon Martinez can play all over the infield."
The injury to LaRoche is the latest in a string of physical problems for the 24-year-old. He previously has had surgery for a torn shoulder labrum and has missed time with lower back spasms.
LaRoche made his Major League debut last May. He came into the season ranked as the Dodgers' best Minor League prospect by Baseball America, but he hit only .226 in 35 games with one home run and 10 RBIs.
LaRoche played for the gold-medal winning Team USA in the IBAF World Cup in Taiwan over the winter, and he had been swinging a hot bat in exhibition games, hitting .350 (7-for-20) with one homer.
Ken Gurnick is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.