-- With shortstop Cristian Guzman struggling both offensively and defensively, Nationals General Manager Jim Bowden instructed the team's player development department to play top prospect Ryan Zimmerman, a natural third baseman, every day at shortstop.

"He has the ability to play both short and third," Bowden said. "That protects us at both positions as we head into September. He's a very special defensive player with great hands and a good arm. He's going to hit. He's going to hit with power. And he's going to be, probably, rushed to the big leagues. That's life."

Zimmerman played third base at the University of Virginia, but Bowden said he saw him take ground balls at shortstop during a workout. Bowden, not shy in favorable evaluations, wrote in his ensuing report that Zimmerman "could be Cal Ripken at shortstop and somewhere between Scott Rolen and Brooks Robinson at third base."

Zimmerman entered Wednesday hitting .315 and slugging .525 with seven homers and 27 RBI in 49 games for Class AA Harrisburg. On Tuesday, Manager Frank Robinson blasted Guzman's shaky defense -- six errors in the previous eight games.

In Wednesday's 4-3 loss to the Phillies, Guzman responded by hitting two balls hard, one for a double, and making a sterling play on defense, picking a throw from catcher Brian Schneider out of the dirt and turning a double play.

Halama Gets the Call

The Nationals shuffled their pitching rotation following Tuesday's rainout, calling up left-hander John Halama from Class AAA New Orleans. Halama will likely start the final game of this 13-game road trip on Sunday in New York against the Mets.

Right-hander Ryan Drese was the starter Tuesday against the Phillies, but the game was called in the bottom of the first and rescheduled as a day-night doubleheader Thursday. Drese will start the night game instead of on Sunday.

The Nationals signed Halama to a minor league contract on Aug. 5. He appeared in 30 games for Boston, going 1-1 with a 6.18 ERA. In two starts with New Orleans, he was 1-0 with a 1.13 ERA.

The Nationals optioned outfielder Brandon Watson to New Orleans after Wednesday's game.