In his four starts with Class A Potomac while rehabbing from elbow reconstruction surgery, he has done nothing to prove otherwise.
The 24-year-old pitched four scoreless innings in his most recent rehab assignment on Wednesday. In facing the Salem Red Sox, he gave up only two hits with zero walks and four strikeouts.
Of his 47 pitches, 34 were strikes. Zimmermann also consistently had his fastball between 92 and 94 mph.
He said he is going to pitch one game in Class A Hagerstown next week before moving to Double-A Harrisburg. Zimmermann expects to pitch five innings in each future Minor League start.
After Wednesday's game, Zimmermann said he was pleased with his outing and that his control -- his biggest issue early on with Potomac -- was back to where he wanted it.
"My arm felt great today," he said. "The control has always been there for me, but I figured after surgery, I wouldn't have the control that I used to have. I think we are pretty much there right now."
The right-hander began the game exactly as he had hoped, retiring all three batters after only 10 pitches.
In the second inning, Zimmermann allowed a leadoff single to center field to Tim Federowicz. It wasn't a problem, though, as he calmly got Alex Hassan to ground into a 6-3 double play. Zimmermann then struck out Jon Hee looking.
After striking out Jason Place to begin the third inning, Zimmermann gave up a single to center field to Drew Hedman. But similar to the second, Ryan Dent grounded into a 6-3 double play to end the inning.
His most impressive inning might have been the fourth. Zimmermann retired the side -- including two strikeouts -- in 14 pitches.
"I was mixing it up, trying to get some changeups in there," he said. "I was working guys away and then coming hard in. They fouled a couple pitches off, and finally I got enough on it to get it past them inside or they just watched it."
Potomac pitching coach Paul Menhart said Zimmermann has improved dramatically since being sent to Class A in early July.
"He had a feel for everything tonight," Menhart said, "the feel for the changeup, the spinning of the breaking balls and the command of the fastball. Everything across the board was improved this evening."
Many project Nationals rookie right-hander Stephen Strasburg and Zimmermann as the future 1-2 pitchers in the rotation.
As a rookie in 2009, Zimmermann started 16 games and went 3-5 with a 4.63 ERA. But elbow discomfort eventually led to Tommy John surgery that was supposed to keep him out for up to 18 months.
Zimmermann, though, recovered ahead of schedule, and the Nationals believe he has the potential to start again in the Major Leagues by late August or early September.
So far in his short stint with Potomac, he has proved just that. In four games against Class A competition, Zimmermann has not allowed a run or walk in 13 innings.
Potomac catcher Derek Norris said he felt Wednesday's performance was Zimmermann's best to date.
"All four of his pitches were good," Norris said. "His changeup is developing a lot better -- I know that was his one pitch that he was really conscious about. I saw that progress a lot since his last start. His slider is coming along as well, and his curveball has been there from the start."
Zimmermann said he has no specific date or goal on when he wants to make it to Washington. Getting back to his original form is all that matters right now.
"I just want to keep pitching the way I've been pitching," Zimmermann said. "Time will tell. They'll let me know when it's time to come up."
Greg Rosenstein is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.