Zach Walters. (David J. Phillip / AP)

The Nationals called up five more September reinforcements on Tuesday, including power-hitting infield prospect Zach Walters. Also arriving were  left-handed reliever Xavier Cedeno, utilityman Jeff Kobernus, and outfielders Corey Brown and Eury Perez, all of whom had already made their major league debuts.

Walters is the lone newbie. He was in big-league camp for the past two years, but this is the first time he will don a Nationals uniform during a major league game.

“It’s humbling,” he said, as he put on the final touches of his uniform in the visitor’s clubhouse at Citizens Bank Park in Philadelphia. “I’m just a kid from a small town. I worked hard but to be here it’s part of a dream come true. I know I’m sure you guys have heard, ‘Dream come true,’ but I always dreamed of winning a World Series, so I think this is an amazing opportunity I have.”

Walters, 23, joins the Nationals after an all-star season with Class AAA Syracuse. The Nationals acquired him from the Arizona Diamondbacks in July 2010 for starter Jason Marquis. In his first three seasons in the Nationals minor league system, Walters hit a combined 23 home runs. After Nationals Manager Davey Johnson urged Walters to improve his power stroke during spring training, the switch-hitting infielder smacked 66 extra-base hits this season, including 29 home runs.

“When Davey said that in spring training, I knew that was what I really wanted to work on,” Walters said. “Because I didn’t feel like I’d ever thought about hitting for power. Just put the ball in play, run around, throw the ball. That’s all I ever thought about. But Davey says something, you better do it twice.”

It didn’t begin instantly. Walters began his first full season at Syracuse slowly, hitting .227 in April and .202 in May. Early in the season, he sat down with Syracuse hitting coach Troy Gingrich and asked for help on the changes. He wanted to make more contact; he had been sacrificing too much of that while trying to produce power. He wanted to balance his own swing with what the Nationals wanted for his development. He worked to stand more upright, spread out his body and balance his weight. Walters jokingly referred to his stance as “Bambi on ice.”

Johnson told General Manager Mike Rizzo before the season that Walters would have a great season, and he hasn’t disappointed. But Walters also understands he needs to improve his plate discipline — he posted a .286 on-base percentage and struck out 134 times in 487 at-bats. He possesses a strong throwing arm but made 31 errors in 104 games at shortstop.

“But I like his hands,” Johnson said. “I think he’s got a great future.”

Walters may see little playing time, if any, during his stint with the Nationals. He is a part of the future, but he wants to be a “fly on the wall” this September. Asked if Johnson had informed him of his role, Walters quipped: “We talked about Powerade mixes. Blue Powerade mixes. Guys like the seeds in alphabetical order. I think the scrubbing bubbles are somewhere around the corner.”

Because the Nationals still believe they can make a push for the a wild-card spot with 25 games left to play, the call-ups are not expected to see much playing time.

“It’s not going to be a priority,” Johnson said. “And they understand that. If we continue to play like we have three of the last four, they’ll be getting a lot of playing time. But I don’t expect that to happen. I expect us to play better and make this a pennant race.”

The Nationals would like to evaluate Cedeno because of a need for left-handed relieving depth. The two left-handers in the Nationals bullpen have struggled; left-handed batters are hitting .321 off Fernando Abad. Ian Krol has held left-handed batters to a .200 average but has a 5.91 ERA since the all-star break and was optioned to Syracuse for two weeks. Tyler Clippard has held left-handers to a .155 average but is too valuable a reliever for that limited role. Cedeno held left-handers to a .164 average with Syracuse.

“I love this time of year when you expand the rosters,” Johnson said. “I know some managers don’t but I think this has always been good for the young talent to get some major-league experience and get a chance to play in some games.”

Danny Espinosa, as expected, was not among Tuesday’s call-ups and was told he would not be called up this September. Even though Espinosa could serve as a valuable defensive replacement, the Nationals have talked about September call-ups as a reward for performance. And Espinosa hasn’t made the needed adjustments, according to Johnson. He also believes Espinosa would not benefit from infrequent playing time in the majors.

“I consider Danny an everyday player,” Johnson said. “I love Danny. The situation we’re in right now and the struggles he’s had this year, I wouldn’t want him to be sitting around here not playing. I don’t think he would’ve been happy with it. I know he’s got a lot of talent. It would be nice to put him in to pinch-run and occasionally pinch-hit and provide some defense. He could do that. But it’s so far down his role, that would be such a lesser role than what I’ve always envisioned for him while he’s been here. So he could be more mad at me because I did not want him here and not have him in the lineup.”