Zach Walters started Saturday night in left field. (Alex Brandon/Associated press)

Over the past few weeks since he was called up following Ryan Zimmerman’s thumb fracture, Zach Walters has been shagging flyballs in the outfield. Two weeks ago, he even went through outfield throwing drills under the watchful eye of Manager Matt Williams and General Manager Mike Rizzo. Walters, a Nationals infield prospect, was again in the outfield throwing home and to third base during early drills before Saturday’s game.

But this time it was different. Walters was warming up for his first professional start in the outfield. In 465 minor league games, the 24-year-old has played only one game in the outfield, and it was in right field in 2012 with Class A Potomac. Walters has played in 26 major league games, but started only once and it was at shortstop. On Saturday, he was the Nationals starting left fielder.

“If you’re athletic enough to play shortstop, you’re athletic enough to play the outfield,” Nationals defensive coordinator/advance coach Mark Weidemaier said.

The Nationals are without left-handed first baseman Adam LaRoche on Saturday as his quad strain is evaluated, and the ripple effects were felt throughout a roster already lacking an left-handed-hitting Bryce Harper. With LaRoche out, Kevin Frandsen, who would likely have started in the outfield, has to play first base. Facing a right-handed stater in Sonny Gray, Nate McLouth is starting in right field and Jayson Werth is the Nationals’ designated hitter.

Williams wanted a left-handed hitter in the lineup, so they turned to Walters, who is 3 for 18 with three home runs this season mostly as a pinch-hitter.

“Zach’s got the ability to change the game with one swing,” Williams said. “He’s been working out there. He’s played out there a little bit. He’s okay as far as his comfort level out there. He provides a switch-hitting opportunity and certainly power from the left side. … We’ve talked to him about the possibility of something like this. It’s not ideal for us to put Zach out there but he does give us more left-handed at-bats against a really tough pitcher.”

Walters is athletic, has speed and power. Last season, Baseball America rated Walters as the best infield arm in the Nationals’ minor league system. First base coach Tony Tarasco, who also coaches the outfielders, spoke highly of Walters’ athleticism.

“He’s a wonderful athlete,” Tarasco said. “He’s ambitious about wanting the baseball. He’s done a great job with all the work we’ve done. He’s done a great job of finding all the extra work making sure he shags in all three spots in the outfield. He’s thrown the ball really well. He’s about as prepared as you can be.”

Walters isn’t completely new to the outfield. He shagged flyballs in the outfield in the Arizona Fall League in 2011. He was primarily an infielder at the University of San Diego, but he also played center field one year. The issue won’t be Walters’ ability to chase down flyballs and catch them. Rather, the concern will be situational defense and lack of experience.

“When it comes down to running flyballs and stuff like that, that’s when the athleticism shows,” Tarasco said. “Playing the outfield is more than just running around. My grandma can catch a flyball but she doesn’t know what to do with it after she catches it. I think that’s where that’s going to be the biggest part for him, knowing mentally where he’s going to throw the baseball and in what situations.”