ARLINGTON – A few hours before the start of a crucial four-game series with the front-running Rangers in the American League West, the three-time reigning division champion Angels moved to bolster their offense on Thursday.

Alberto Callaspo, a switch-hitting infielder expected to spend most of his time at third base, was acquired from the Royals in exchange for pitchers Sean O’Sullivan and Will Smith.

Callaspo, 27, is hitting .275 for the Royals this season after batting .300 in 2009 with a career-high 11 homers and 73 RBIs. He has eight homers and 43 RBIs this season. He signed with the Angels in 2001 as a teen in Maracay, Venezuela, and was trade to Arizona for pitcher Jason Bulger. The Royals acquired Callaspo for Billy Buckner.

“He’ll be a boost,” Angels manager Mike Scioscia said. “He can definitely play third base every day. We’ll see how some of the pieces fit. The work he does in the batter’s box is special. He can hit. He’ll  help fill the void Kendry Morales’ departure created.”

O’Sullivan, 22, held the Yankees to two runs on two hits in six innings in a victory on Tuesday night at Yankee Stadium. The Angels were 9-2 in the big right-hander’s 11 starts over the past two seasons. The San Diegan was a third-round pick in the 2005 First-Year Player Draft.

Smith, a 21-year-old native of Georgia taken in the seventh round of the 2007 First-Year Player Draft, is a 6-foot-5 left-hander in his third professional season. He has pitched for high Class A Rancho Cucamonga and Double-A Arkansas this season and is considered to have high upside.

“This was not by any means an easy decision on Tony’s part,” Scioscia said, referring to general manager Tony Reagins. “It’s not easy to put Sean O’Sullivan or Will Smith in a deal unless we were getting a player who can help us.”

Callaspo is expected to join the Angels on Friday night. O’Sullivan, who had been scheduled to pitch Sunday’s series finale, likely will give way now to Trevor Bell. Scott Kazmir (shoulder fatigue) is not eligible to come off the 15-day disabled list until Monday, and it is unclear when he’ll be ready to rejoin the rotation.

“It’s weird,” O’Sullivan said, minutes after being informed of the deal by Scioscia. “It’s the first time I’ve ever been traded. The goal is to go to a place where they need you, want you, so you can show you’re capable of playing at this level.

“I’m trading red for blue. That’s all I know right now.” — Lyle Spencer