The moves mean that the team will open the regular season with 12 pitchers, three catchers, six infielders and four outfielders.
General manager Jim Bowden called the final cuts "devastating," because he felt Casto and Rivera performed well enough to make the team.
"Tough decisions -- both of those guys deserve to be on the team," Bowden said. "They won't let us carry 27 players. They have options, other guys didn't. They will both be back. We all know how many guys we carry. Those guys in particular will spend a lot of time up here this year. They deserve to be here."
Casto is the bigger name of the two. He made it tough on the Nationals to demote him. They loved the fact that he was patient at the plate and could play several positions -- left and right field, third base and first base -- very well. It got to a point during the spring where some members of the Nationals' front office wanted Casto to be the starting left fielder over Ryan Church. Casto would end up hitting .280 with five RBIs this spring.
In the end, however, it came down to Casto having a Minor League option or putting Church or outfielder Chris Snelling on waivers. Doing the latter was never going to happen. Plus, manager Manny Acta promised Church that he was the starting left fielder no matter what he did during Spring Training. And, despite being in a recent slump, Snelling's hustle and determination made him a lock on the 25-man roster.
"[Kory] handled it very well. It was something that I talked to him about during Spring Training and he understood the situation," Acta said. "We also made sure that he knew that he is down there for us to grab anytime."
The only way Casto was going to make the roster was if center fielder Nook Logan started the season on the disabled list. Logan suffered a right groin strain last Saturday against the Cardinals and the team thought he would be out at least two weeks, but Logan made a quick recovery and started playing again Friday against the Orioles. With Logan healthy, it made Casto expendable. Casto received the news about his demotion from both Acta and Bowden on Saturday morning.
"This kid is a first-class kid. He totally understands that he'll be back," Bowden said. "You are not going to find a better person than Kory Casto. He is a gamer -- hard-nosed kid. I'm a Kory Casto fan. He is going to hit. He knows how to approach at-bats, he knows how to take pitches, [and] he knows how to foul off pitches. He is a professional hitter.
"First and third with one out and Kory is at the plate, I feel good that the run is going to come in. I know he is going to put the bat on the ball and figure out how to get that run in."
Casto said he saw the demotion coming the last few days, but he is glad that he showed the Nationals that he could play Major League Baseball. Casto will go to Columbus on Sunday and be the everyday left fielder. He also will play third and first.
"My goal was to get them to see me and know that I was ready to play in the big leagues," Casto said. "I feel after what I did this spring, I showed that I could play different positions. No matter where they put me, I worked hard every day to get better there -- even if it was some place I never played. At the plate, I had good at-bats, I hit the ball hard, [and] the outs were good outs. I didn't strike out a lot. That's what I came here to do from Day 1."
Rivera, who appeared in 54 games for the Nationals in 2006, was battling right-hander Jesus Colome for the final roster spot in the bullpen. Unfortunately for Rivera, Colome didn't have options and pitched well this spring. Colome was signed to a Major League contract and placed on the 40-man roster early in the week. Although he had a 6.14 ERA, Rivera gave up one run in his last 5 1/3 innings.
"I'm seeing the demotion as a positive," Rivera said. "I have to keep working hard. That's all. I don't have a problem."
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.