MIAMI — Team USA heads into the semifinals of the World Baseball Classic with one glaring flaw: It needs someone to play first base.

Kevin Youkilis left the tournament Wednesday because of a sprained left ankle, the latest in a wave of injuries that has plagued the United States squad. His absence left the Americans without many options at first as they prepare for the final four in Los Angeles this weekend.

Youkilis is unlikely to play again in the WBC, manager Davey Johnson said. Team USA's cleanup hitter and first baseman returned to Boston Red Sox camp in Fort Myers for an MRI.

Rules for replacing players leave the Americans hard-pressed to come up with a substitute for Youkilis, Johnson said.

"These things need to be addressed" in the future by WBC officials, Johnson said. "You just can't have this becoming a circus, playing guys out of position."

Adam Dunn was slated to play first base Wednesday night in Team USA's game against Venezuela to determine seedings for the semifinals. But the Washington Nationals would prefer Dunn remain in the outfield, Johnson said. Utilityman Mark DeRosa is on the roster, but the Cleveland Indians don't want him playing first, Johnson said.

The only first baseman on the list of potential roster replacements is Derrek Lee of the Chicago Cubs, who has been hampered by a sore thigh.

Earlier injuries ended the tournament for infielders Dustin Pedroia and Chipper Jones and reliever Matt Lindstrom. Outfielder Ryan Braun is sidelined with a sore right side but expected to be available for the semifinals.

Youkilis had been bothered by his injury for several days, Johnson said. The Red Sox slugger homered Tuesday, and the Americans advanced with a three-run rally in the ninth inning to eliminate Puerto Rico 6-5.

Only 13,224 fans saw the thrilling finish, and many were rooting for Puerto Rico. While injuries and modest fan support have been the disappointing parts of the WBC for Team USA, the comeback left players raving about their experience with games that matter in March.

"I can't imagine a player being asked to do this and being healthy and turning it down," said right-hander Jake Peavy, who will start for the Americans in the semifinals. "To play in a game like that — we have guys here who have won World Series championships who said that was the happiest and most memorable moment of their career last night."

David Wright hit a game-winning, two-run single with one out in the ninth, then was buried in a pile of teammates.

"It was a dream come true to be mobbed and tackled by some of the best players in the game," Wright said. "That's a pretty cool feeling."

The semifinal berth is the first for Team USA, which was eliminated in the second round of the inaugural WBC in 2006.

"Baseball is our game. We've set the example of excellence, and a lot of countries have come a long way to catch up," Johnson said. "Three years ago we didn't show so good. Everything I read was that we weren't prepared. To come back and win here and prove everybody wrong, representing your country, it couldn't get any better than that."