Rosenbaum promoted to Potomac

Daniel Rosenbaum’s impressive season is moving to a new location.

The left-hander, who put himself on the Nationals’ organizational map at Single-A Hagerstown, was promoted to Single-A Potomac this morning and will pitch for the P-Nats tonight against Salem.

Rosenbaum has a 2.32 ERA in 18 starts, and has impressed Nationals officials with his control and ability to change speeds. He’s got a 2.45 groundout-to-flyout ratio this year, and has walked only 28 batters.

There wasn’t much more for Rosenbaum to do at Hagerstown, and at Potomac, he’ll get a chance to round out his three-pitch repertoire in a smaller league with more advanced hitters.

Nationals player development director Doug Harris said the Nationals have been drilling the importance of the changeup with Rosenbaum, as they do with many of their young pitchers. But the problem comes in when a pitcher’s as dominant as Rosenbaum has been at Hagerstown: there’s no need to throw a changeup when a fastball and a breaking ball are enough to get lower-level hitters out.

“When you’re in the South Atlantic League, he’s able to get by with two pitches,” Harris said. “When we put him in a smaller league like the Carolina League, he’s going to be forced to make that adjustment, hitters are going to tell him when he has to throw (the changeup) without us having to do it.”

When Harris was a pitching prospect in the Royals’ system, he would occasionally be asked to pitch without a breaking ball in instructional league games. That’s something Harris said he’s talked with minor-league pitching coordinator Spin Williams about doing with the Nationals’ young pitchers in instructional league, or early in spring training, so they can learn changeup command.

“Most college pitchers don’t use their changeup much,” Harris said. “When guys are having success, it’s difficult to force that. We’re going to call it, but (now) we’re forcing him to throw it with conviction.”

A 22nd-round pick in 2009, Rosenbaum is a typical Nationals groundball specialist. He’s a favorite of Nationals player development director Doug Harris, and could start next season at Double-A Harrisburg if he finishes the season well.

To make room for Rosenbaum, the Nationals activated left-hander Evan Bronson from the disabled list and moved him from Potomac to Hagerstown. They also gave right-handed reliever Jesse Estrada his release.