(Photo by Jonathan Newton / The Washington Post)

UPDATED, 6:40 p.m.: Nationals reserve outfielder Nate McLouth landed on the 15-day disabled list after he underwent an MRI exam Monday morning on his ailing right shoulder. The Nationals knew McLouth would require a disabled list stint regardless of the test’s outcome, and they summoned outfield prospect Steven Souza Jr. from Class AAA Syracuse to take his roster spot.

McLouth has played through inflammation in his right shoulder of late, an ailment the Nationals and McLouth have kept private until now. Williams believes McLouth likely hurt his right shoulder when making a diving attempt for a triple in the seventh inning of the July 28 game in Miami.

McLouth didn’t immediately feel any shoulder discomfort but began feeling soreness in later days, Nationals Manager Matt Williams said. McLouth received a cortisone shot on Monday and will be shutdown from all activity for the next five days, General Manager Mike Rizzo said.

“After that, he should start gradually returning to play and play rehab games and get some at-bats,” Rizzo said.

In July, McLouth started only one game and received just 16 plate appearances. In the first year of a two-year, $10.5 million contract, McLouth is hitting .173/.280/.237 with one home run.

“He should be fine, though,” Williams said. “It’s nothing serious. He should be fine.”

Souza has blossomed into one of the Nationals’ top prospects. This season at Syracuse, he leads the International League in hitting, on-base percentage and slugging with a .354/.435/.601 batting line. He’s also tied for the league lead with 70 RBI in 91 games and has smashed 18 homers. He has also stolen 24 bases.

With Bryce Harper, Jayson Werth and Denard Span entrenched in the outfield, Souza will join the Nationals’ bench. He can play all three outfield positions, but he fits best in the corner spots. Williams said he hopes to get Souza occasional starts, perhaps in place of Span or Werth, both who have played a lot.

“I would anticipate Steven will be in the lineup from time to time starting, certainly coming off the bench to pinch-hit and running the bases,” Williams said. “He adds a lot of things to our club.”

Souza said he was pulled from the first inning of Sunday’s Syracuse game after a single. The coaching staff originally told him, as a joke, that he was pulled from the game for not running out a groundball. Then he received hugs from his teammates. He was told to wait and see. It wasn’t until after the bus ride back from Scranton that Souza was told that he was heading back to the major leagues.

“I had to wait until the bus ride, so two hours on the bus, my family is texting me and I had to tell them, ‘I don’t know what’s going on right now,'” Souza said. “They told me when I got back and I was excited.”

The biggest challenge for Souza will be adjusting to the sudden drop-off in playing time. He first learned to handle that when he played once or twice a week during the past Arizona Fall League. And, as he did during his previous brief major league stints, he will pick the brains of veteran bench players Scott Hairston and McLouth.

“Any way that I can help this team win a World Series that’s what I’m here for,” Souza said. “Whatever role they put me in, I’m ready to accept the challenge.”

“He’s had an incredible year and we don’t want him to lose that feeling,” Williams added. “It’s a tricky one, but we’ll try and manage it as best we can.”

Before Monday’s game, Souza took grounders at first base, which he has done on infrequent occasions at Syracuse. Souza was originally a third baseman in the minor leagues before shifting to first base. He last played first base in 2011 at Class A Potomac and made 11 errors in 117 games. He moved to the outfield the next season.

Souza also took grounders at first base during spring training. Williams said he considers Souza the emergency first baseman behind backup first baseman Kevin Frandsen.

“I played infield my whole life,” Souza said. “It’s kinda like riding a bike as far as getting groundballs. Getting live balls and stuff may be a little different. But yeah, I’m excited to take groundballs. I’ve been taking them every now and then down there. In spring training, it got going there. I’m excited for it.”

On Monday afternoon, another Nationals outfield prospect, Michael A. Taylor, was promoted from Class AA Harrisburg to Syracuse, a ripple effect of Souza’s call-up. Taylor, rated by Baseball America before the season as the Nationals’ seventh-best prospect, was tearing up the Eastern League. He is hitting .313/.396/.539 with 22 home runs and 34 stolen bases, along with stellar outfield defense.

“He’s found his power stroke,” Williams said. “He’s had a really good year. His pitch recognition is improving on a daily basis. He is as good as I’ve seen in the outfield, he can cover extreme ground out there. He makes it look really easy in center field. He’s a young hitter still, but he’s made those adjustments and he continues to make them. Now he’ll have another test, but that’s part of the progression that he has to go through. He’s had a great year. Power numbers are good and he’s getting a lot of basehits.”