WASHINGTON -- Before Thursday’s series finale against the Cardinals at Nationals Park, Washington manager Dave Martinez made a point to have a reassuring conversation with Carter Kieboom, the organization’s top prospect thrust into the big leagues at 21 years old as the starting shortstop. Martinez gave Kieboom a day off Thursday, a “mental break” just six games into his Major League career, to help clear his head.

A week ago Thursday, Kieboom woke up to several missed calls from his Triple-A Fresno manager informing him he was headed to the big leagues. In his Major League debut last Friday, he launched a game-tying home run in the eighth inning. Two days later, he hit his second homer. Entering Thursday, however, Kieboom was hitting just .136/.240/.409 in 25 plate appearances, striking out 36 percent of the time and coming off a rough game defensively on Wednesday night. He entered to pinch-hit in the seventh inning of Thursday's 2-1 victory over the Cardinals and struck out.

What a difference a week makes.

“For a 21-year-old kid,” Martinez said, “it’s been a whirlwind.”

The Nationals promoted Kieboom because they believe he can handle it. They raved about his poise on and off the field. But Martinez also believes the game has sped up on Kieboom in recent days.

On Wednesday night, he misplayed a grounder with the infield in and then made an error that led to a run in the eighth inning. Martinez noticed his swings getting bigger as the night went on, seemingly trying to erase his mistakes in the field with one swat.

“I’m not going to sugarcoat anything,” Kieboom said Wednesday night. “Yeah, I should definitely be doing better, but at the same time it's not something I can sit here and hold my head down about and carry onto the next day and next at-bat.”

Kieboom earned this callup in part because of his bat. He started the year with a 1.142 OPS and three home runs in 18 games with Fresno after a strong spring at the plate. The Nats knew Kieboom’s offense was going to be ahead of his defense -- in fact, it’s the reason general manager Mike Rizzo gave for why he did not promote Kieboom right away when Trea Turner went on the injured list on April 3 -- but they also wanted to add a spark to their limping lineup.

With Turner sidelined, the Nationals are still committed to giving Kieboom an opportunity to prove he is ready to play in the big leagues every day. So Martinez wanted to reassure Kieboom by giving him a mental break Thursday, while keeping him available to pinch-hit and having him ready to return to the starting lineup the next day.

“I know what I'm capable of doing,” Kieboom said. “I know I can hit. I know I can play defense. It’s not a matter of me having confidence. I know I can do all those things.

“It's just, it's one of those things where baseball's a crazy game. You can be the man on the moon for a while and then next thing you know you haven't swung a bat before.”

Soto MRI comes back clean

Juan Soto received an MRI on his back Thursday, after being scratched from the lineup for the second consecutive game with back spasms. The results of the MRI came back clean, but Soto is day to day as he battles the back issue that has kept him out of the starting lineup the past two games.

Soto was initially in the starting lineup and playing left field after arriving Thursday with his back much improved. But he tried to hit in the cage prior to the game and the Nationals had to remove him from the lineup for the second game in a row.

“We thought he could play,” Martinez said. “He did everything, warmed up, and then he went to go hit in the cage and got him back."

Adam Eaton shifted to left field, Victor Robles shifted to right and Michael A. Taylor came off the bench to play center field in Soto’s absence. This is the first time Soto has dealt with back issues, and the Nationals are going to exercise extra caution with their 20-year-old phenom.

“He's a huge part of our lineup, but we want him for longevity. We don't want to risk something else happening," Martinez said.

Worth noting

It’s been about four weeks since Turner broke his right index finger, and he continues to do as much activity as he can to stay in shape while the finger heals. He will travel with the team on this next 10-day road trip, and Martinez was hopeful he can begin swinging a bat later this week.