Zimmermann allowed one hit over 4 2/3 innings Saturday night as the Harrisburg Senators cruised to a 10-0 rout of the Portland Sea Dogs.
"I know it's a process. It'll be five [innings] and done the rest of the year, but I'll stick to the program," he told the Harrisburg Patriot-News. "You just don't want to screw anything up."
Zimmermann did not allow a hit until Luis Exposito singled in the fourth inning. He struggled with his control, walking two batters, hitting another and uncorking a wild pitch. It was just the second time the 24-year-old right-hander has pitched into the fifth since undergoing ligament replacement surgery last August.
"It feels like a brand new arm," Zimmermann told the newspaper. "Before, I wouldn't even be ready by the fifth day, but now I feel like I could pitch again on my bullpen day."
The former second-round pick made 16 starts as a rookie for the Nationals last season, going 3-5 with a 4.63 ERA. He led the team with 92 strikeouts over 91 1/3 innings.
Zimmermann began his road back to the Majors on July 3 with Class A Advanced Potomac. He threw 13 innings without allowing an earned run in four Carolina League starts, then surrendered six runs on seven hits over five frames for Class A Hagerstown on Monday.
Former Major Leaguer Chuck James (6-0) came on in the fifth and did not allow a hit until Nate Spears singled with two outs in the eighth. The left-hander struck out four and walked two over three innings before Zach Dials gave up one hit and fanned one while recording the final four outs.
The Senators scored early, plating two runs in the first, three in the third and five in the fourth. Josh Johnson went 3-for-4, scored three times and fell a triple shy of the cycle, while Michael Martinez drove in a pair of runs. Leadoff hitter Danny Espinosa scored twice.
Casey Kelly (3-5) took the loss for the Sea Dogs after surrendering seven runs on seven hits over three innings. The former first-rounder has a 5.40 ERA in his first season as a full-time pitcher after spending half of the 2009 campaign as a shortstop.