BALTIMORE -- Pitching hasn’t been the Nationals’ only area of need as of late. Their infield depth shrunk this weekend when Jordy Mercer, who had been starting at third base with Starlin Castro on administrative leave, was placed on the 10-day injured list because of a left calf strain.

Three months after his last stint in the Major Leagues, Carter Kieboom made his return to the Nats on Saturday.

“I feel so ready,” Kieboom said. “Just really excited to be a part of it.”

Kieboom made his first start of the season on Sunday when he was tabbed at third base in the Nationals’ 5-4 walk-off loss vs. the Orioles at Camden Yards. He had pinch-hit and had two plate appearances on Saturday, after which manager Dave Martinez complimented his bat speed, and Martinez wanted to get him consistent looks on Sunday facing a left-hander in John Means. Kieboom went 0-for-3 with a walk in a six-pitch plate appearance to load the bases in the eighth inning.  

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The 23-year-old was in the thick of the action on the final play of the afternoon, which clinched the Orioles’ series sweep. After closer Brad Hand surrendered the game-tying run in the bottom of the ninth, Ramón Urías sent a fielder’s choice to third base. Kieboom swiftly fielded it and fired off to Tres Barrera at home plate. After an official review, a sliding Ryan McKenna was ruled safe for the walk-off run. 

“He hit some balls hard to center field, he had a great walk there at the end and I thought he made a nice play [on] the last play of the game. [McKenna] just got in there,” Martinez said. “I thought [Kieboom] did well.”

The Nationals gave Kieboom the opportunity to earn the starting third base job in the past two Spring Trainings. Last year, he transitioned from shortstop to third and ended up platooning at the position with veteran Asdrúbal Cabrera for 33 games while struggling at the plate. 

At the start of this season, the Nationals opted to shift Castro to third. Kieboom was optioned to Triple-A Rochester to build up his reps and continue his development. He has increased his fielding percentage to .973 in Triple-A, up from .966 in 2020 with the Nats. 

“I got really comfortable with it,” Kieboom said of playing third base. “I enjoy it over there. You get the hot shots or the slow ones. So it's fun, [it] keeps you on your toes and I treat it like a goalie almost. … Catch it first, then take care of the rest. That's just my game plan over there: get down and see it off the bat.”

Kieboom was out with a knee injury (which he described as a “freak accident” while stretching) when Castro was placed on administrative leave earlier this month. But by the time Mercer was sidelined, Kieboom was healthy -- and playing well. He had hit safely in seven of his last 11 games, including two home runs and eight RBIs, prior to the callup. Overall, he was hitting .236/.376/.385 with 26 runs, 23 RBIs and 26 walks on the season with Rochester. 

“I've gotten back to my swing, and it's been nice,” Kieboom said. “Now I can stick with my approach again. It’s a lot easier to hit a baseball when you're not thinking about your swing.”

Kieboom had been promoted to the Major Leagues before, but this callup felt different thanks to a new outlook. 

“The biggest takeaway I've had this year versus all the other years is, no matter what the outcome is, I’ve just been able to have fun and show up every day and smile,” Kieboom said. “And that's all I can do.”

The Nationals will evaluate third base, along with all other positions, with less than a week remaining before the Trade Deadline on Friday. In addition to Kieboom, utility man Josh Harrison and infielder Alcides Escobar are among those on the active roster who also can play third base.