"He's got a little bit of a fever, nothing major," said manager Fredi Gonzalez.
McCann was in the original lineup batting fifth before Gonzalez knew about his condition. McCann went 0-for-3 with a walk against the Reds in Monday night's 4-1 loss. He's hitting .254 with six home runs and 24 RBIs this season.
Tuesday's start was Ross' 10th of the season, including Sunday at Tampa Bay, when he hit a solo home run off of David Price in Atlanta's 2-0 victory. Overall, Ross is hitting .303 in 12 games played. Ross is in his 10th season in the Majors and his fourth with the Braves.
Gonzalez jumbled the lineup with McCann out; Jason Heyward batted fifth, Juan Francisco sixth and Ross seventh.
"Brian is such a big presence in our lineup, and I don't want to be that weak link when he gets a day off," said Ross. "I just try to contribute to the winning."