Nationals manager Matt Williams said Zimmermann was being sent back to Washington ahead of the team and being given fluids. He said the pitcher, who went 19-9 with a 3.25 ERA last year, was feeling fine Wednesday but took ill after the game.
"He's running a fever. He was up all night throwing up," Williams said. "Evidently he's feeling pretty under the weather."
It was to be Zimmermann's first start of the season after a stellar spring training in which he allowed one run on 11 hits and a walk in 18 innings. He also struck out 15 batters during Grapefruit League action.
Roark, a 27-year-old right-hander, was 7-1 with a 1.51 ERA in five starts and nine relief appearances as a rookie last year, striking out 40 and walking 11 in 53 2/3 innings.
Williams hopes Taylor Jordan is ready to pitch Friday on short rest. While his last exhibition appearance was March 24, he threw a simulated outing this week and would be starting on short rest.
Washington's second option would be a series of relievers.
"If he's not ready to go tomorrow, then that's going to have to be our option," Williams said.
The Associated Press contributed to this story.