Casto, expected to be the everyday third baseman at Columbus, had known since the beginning of Spring Training that he wasn't likely to be placed on the 25-man roster because of his poor 2007 campaign.
Casto started the '07 season with Triple-A Columbus, but didn't stay there long. He was recalled to the big leagues before the second game of the regular season because of an injury to center fielder Nook Logan.
But Casto struggled. He stopped pulling the ball and showed little patience as a hitter, going 7-for-39 (.179) in his first stint with the Nationals. He later went hitless in 15 at-bats in his second stint.
The poor showing carried over at Columbus, where he hit .246 with 11 home runs and 55 RBIs.
Meanwhile, the Nationals were impressed with Shell's breaking balls this spring, but he was hit hard in his last outing against the Indians on Friday. He gave up three consecutive home runs in that game.
In a meeting after Saturday's loss to the Dodgers, manager Manny Acta and pitching coach Randy St. Claire told Shell his quickest way to the big leagues would be as a long reliever. Shell said he will accept the assignment to Columbus.
"I'm real excited I received this opportunity," Shell said. "I'm just going to go down to Triple-A and work my hardest at filling that role. They said I had four quality pitches and I throw a lot of strikes. They said all positive things."
Munoz, a left-hander, didn't give up a run in 1 2/3 spring innings, but Acta and St. Claire were baffled that Munoz often found himself behind in the count. He walked three hitters this spring.
"He is just walking too many guys," St. Claire said. "He doesn't walk guys in the Minor Leagues, why is he doing it in the big leagues? Maybe he is trying to do too much. It's usually the harder you try, the worse it gets in this game."
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.