The Arizona Diamondbacks made a major move to shore up their ailing bullpen on Tuesday, acquiring reliever Jon Rauch from the Washington Nationals for top infield prospect Emilio Bonifacio.

Rauch was 4-2 with 17 saves in 22 chances and a 2.98 ERA for the Nationals. He struck out 44 and walked seven. The 29-year-old has been filling in as the closer most of the season for the injured Chad Cordero.

Rauch, who has a home in Tucson, was in uniform and available for Arizona in its game against the Chicago Cubs on Tuesday night.

"I'm extremely excited," he said before the game, "coming over to a ball club like this who is winning their division _ tied for first I believe right now. It's a good group of young and old. ... I don't think it could have worked out any better."

Despite a 49-50 record, the Diamondbacks entered Tuesday tied for first in the NL West with the Los Angeles Dodgers.

Bonifacio entered the season as Arizona's No. 6 prospect, according to Baseball America. The speedy 23-year-old second baseman from the Dominican Republic was recalled by the Diamondbacks on July 4, and was 2-for-12 in eight games. He hit .302 in 85 games for Triple-A Tucson with 18 doubles, five triples, a home run, 29 RBIs and 17 stolen bases.

"We have been trying to get Emilio for a long time but they would never give him up," Nationals general manager Jim Bowden said.

Bonifacio, who was optioned to Triple-A Columbus by Washington, was being groomed as Arizona's second baseman of the future. Now the Diamondbacks are faced with either re-signing Orlando Hudson to what promises to be a big-money deal or finding a replacement via trade or free agency. Hudson has a one-year, $6.25 million contract signed just before he would have gone to arbitration.

"Obviously at some point it's a situation we need to resolve _ who's going to be our second baseman next year and beyond," Arizona general manager Josh Byrnes said.

Hudson, a three-time Gold Glove winner, entered Tuesday's game hitting .302.

Bonifacio, the fastest baserunner in the Diamondbacks' system and one of the fastest in all of baseball, played for the World team in the Futures game during the All-Star break.

"He reminds a lot of people of (Mets second baseman) Luis Castillo when he was coming up," Washington manager Manny Acta said. "He's got great character. He's not only a great player but a great person _ the type of person we like to have around here. He's very coachable and the kind of guy you'd like to live next door to."

The 6-foot-11 Rauch is the tallest pitcher in major league history, edging Randy Johnson, his new teammate, by an inch.

The Diamondbacks' bullpen was supposed to be a strength this season but all three late-inning pitchers _ Chad Qualls, Tony Pena and closer Brandon Lyon, have struggled. Lyon gave up five ninth-inning runs in Arizona's 6-5 loss to the Dodgers on Sunday.

"First of all, we have him until 2010," Byrnes said. "Second of all, we've had a lot of leads going into the sixth and seventh innings that we didn't turn into wins."

The Diamondbacks are 6-15 in games that were tied after seven innings. They have lost 16 games when leading after six.

Manager Bob Melvin said Lyon would remain the closer, with Rauch filling in some at that spot but mostly being used as a setup man.

"It just shows that Josh is worried about this year and trying to fill some of the holes that we have this year," Melvin said, "and give us the type of depth in the bullpen that it looks like we need."

Rauch came to the Montreal Expos, who became the Nationals, in 2004 in a trade with the Chicago White Sox. He has appeared in 255 major league games, going 24-17 with a 3.67 ERA and 37 saves over six seasons.

In other moves, Arizona optioned right-hander Connor Robertson to Triple-A Tucson and took over the contract of infielder Jamie D'Antona from the same club. Arizona also transferred outfielder Eric Byrnes (torn left hamstring) to the 60-day disabled list.

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AP sports writer Janie McCauley in San Francisco contributed to this report.