JUPITER, Fla. -- Right-hander Jordan Zimmermann continues to impress the Nationals. On Monday afternoon, he pitched four scoreless innings against the Marlins, striking out six without issuing a walk.

Manager Manny Acta pointed out after the game that Zimmermann's best pitch was his slider. Zimmerman struck out Alejandro De Aza and Emilio Bonifacio on the pitch in the second and fourth innings, respectively.

"He locked up a couple of good hitters in their lineup today. He is not backing down," Acta said. "Not too many Double-A, first-time-in-big-league-camp guys can come out and make a pitch like that. That was literally a big league pitch."

For the spring, Zimmermann, 22, hasn't been scored upon in 12 1/3 innings and has struck out 16 batters. But does his strong Grapefruit League performance mean Zimmermann's a step closer to being in the Nationals' rotation this season? Acta declined to give an answer to that question.

"We have three weeks to go," said Acta. "If he doesn't get anybody out from here to the last day of the exhibition season, then I have to eat my words if I put him in the rotation today. He has a few starts to go. It's clearly not about stuff or whether he can do it or not. We are going to make the decision whether it's going to be the best thing for him or for us. Right now, it looks like it's going to be the best thing for us."

Zimmermann is a man of few words when he is at the ballpark. He is modest when he talks about his success on the mound.

"I just try to work the corners with my fastball," he said. "I just go out there and compete, I guess. I'm going to give it my best shot to make the rotation. It's up to those guys to make the final decision. But I'm going out there to try to put up zeroes every time, to do the best that I can and then leave it in their hands."