The Nationals will not open the 2021 season with Carter Kieboom as their starting third baseman.

Kieboom was among the players optioned to Triple-A Rochester when the team announced roster moves on Saturday afternoon. The 23-year-old had been competing for the starting job this spring, like he did last year. He will begin the season at the Nats' alternate training site in Fredericksburg, Va.

“I know I’ll be back up there with the team and helping them out,” Kieboom said Sunday, following three-plus hours of batting cage work. “So there’s no frustration towards it. I completely understand the move and the decision they made and the direction they went. It’s part of it, and I’ll be better for it.”

This week, manager Dave Martinez decided to explore more options and take a look at shifting Starlin Castro from second to third, where he played 45 games for the Marlins in 2019. The plan is for Castro to be the starting third baseman, once he is cleared to return from a left hamstring strain, while utility man Josh Harrison will start the majority of the games at second.

“With Carter, we really felt strongly about getting him into some type of rhythm and not putting so much pressure on him,” Martinez said on Sunday. “He’s a big part of our future. I’ve said this before and I’ll say it again: We want Carter here, and we want him to be our everyday third baseman in the future. But we want to make sure we get him right and get him in rhythm. He’s a good player. He’s going to be a good player. We want him to just relax and go find his swing and get him going.”

The Nationals added veteran depth to both positions by selecting veterans Jordy Mercer and Hernán Pérez to the Major League roster on Saturday. Martinez noted that Pérez could also be an emergency catcher. (Second baseman Luis García was optioned to Rochester as well.)

“These are two professional guys that give you professional at-bats,” Martinez said. “I could put them anywhere on defense, and they’re going to play really good defense. I love having both those guys.”

Kieboom platooned at third base with veteran Asdrúbal Cabrera in 2020, his first Major League season at the position. He hit .202 with a .966 fielding percentage over 30 starts. Kieboom underwent LASIK surgery this winter and reported to camp having worked on his approach at the plate and footwork.

There still were struggles this spring, though. Kieboom slashed .133/.204/.222 in his first 17 games. He had connected on six hits (including two doubles) compared to 17 strikeouts. Kieboom cited his swing mechanics, and that he notices a difference in a side-by-side comparison with his Minor League at-bats. His outlook is that his success at the plate could click any day, and "that's the exciting part of it."

"I don't have any more frustration to give towards it," Kieboom said. "It's just show up every day, get better and get back to who I was."

The Nationals will play against the Orioles (and possibly other clubs) at the alternate training site, where Kieboom will be able to get reps in before the start of the Minor League season. Both Kieboom and Martinez are thinking ahead to when Kieboom will rejoin the team.

“I told him, ‘This is not the last I’m going to see of him, believe me. We’ll be together for a long time,’” Martinez said. “The biggest thing is to get to work right away and stay positive. … I’ve seen a lot of good players that got sent down early and the next thing you know, they were up in the big leagues nine, 10, 11 years. I told him, ‘You’ll get it. You know you belong here. You know you’re going to play here. Just get ready to go.’”

In addition to optioning Kieboom and García, the Nationals made the following roster moves over the weekend:

• The contract of left-hander Luis Avilán was selected.

• Left-hander Sam Clay, right-hander Kyle McGowin and outfielder Yadiel Hernandez were optioned to Triple-A.

• Right-hander Rogelio Armenteros and left-hander Seth Romero (the Nats’ No. 18 prospect, per MLB Pipeline) were optioned to the alternate training site.

• Right-handers Aaron Barrett, Paolo Espino, Javy Guerra and Todd Peterson, catchers Welington Castillo, Brandon Snyder and Blake Swihart, infielder Adrían Sanchez and outfielders Gerardo Parra and Carlos Tocci were re-assigned to Minor League camp.

• Right-hander Dakota Bacus and infielder Jake Noll were designated for assignment.

• Left-hander TJ McFarland and outfielder Yasmany Tomás were released from their Minor League contracts.