The Washington Nationals have sat players down over the course of the season for myriad injuries, but Wednesday, when third baseman Vinny Castilla had his second day off in four games, Manager Frank Robinson came up with a new malady.

"Tired bat," Robinson said.

When the Nationals signed Castilla, 37, to a two-year, $6.2 million contract in the offseason, they were clear about their expectations. They wanted production similar to his years in Atlanta and Houston, about a .270 average with 70 or 80 RBI, as opposed to the National League-leading 131 RBI he had with Colorado last season.

Castilla's recent slump, though, is threatening his ability to reach those goals. He is 2 for his last 24, dropping his average to .255. He has gone 37 at-bats without an extra-base hit, and his slugging percentage has dipped to .397 -- under .400 for the first time all year.

"I need to get my bat quicker," Castilla said.

Robinson said it's fair to wonder about the erosion of skills for any player over 35.

"He's naturally going down the other side of the hill," Robinson said. "He's not going up the hill. We all know that. That's not a knock at him. It's just a matter of how fast he goes and how productive he can be in the years that's left."

Castilla has five homers and 33 RBI. Jamey Carroll started at third.

Loaiza Better

Esteban Loaiza said his stiff neck felt much better Wednesday, and he believes he'll be able to pitch on Friday at RFK Stadium against Toronto. He plans on throwing at RFK on Thursday's off day to make sure.

"I think I'm better," he said. With that in mind, the Nationals juggled the rotation, scheduling Loaiza Friday, Livan Hernandez Saturday and moving Tony Armas Jr. to Sunday. . . .

Ryan Zimmerman, the Nationals' first-round draft pick from the University of Virginia, began his professional career Tuesday for the Class A Savannah Sand Gnats, going 0 for 4. The Nationals promoted last year's top pick, lefty Bill Bray from William & Mary, to Harrisburg.