In addition to his irregular heartbeat, Dusty Baker suffered a stroke last week and will need another week of rest before rejoining the Reds.
The 63-year-old disclosed the development to his players on Tuesday.
"He's lost a lot of water weight," bench coach Chris Speier said. "He was anxious [to get back]. It was great to see him. Again, he looks really, really good. We're all anxious to get him back in charge."
For now, Speier will manage the three-game series against the Brewers and a three-game series in Pittsburgh over the weekend.