Baker, who was admitted overnight after complaining of trouble breathing, is expected to miss the team's game Thursday, as well, The Cincinnati Enquirer reported.
"Basically, he had a heavy feeling," trainer Paul Lessard said. "He wasn't breathing well. We decided to look into it further. He has a history of an irregular heart beat. It doesn't really bother him. He was kind of pooh-poohing it."
He left the visiting locker room at Wrigley Field 2 1/2 hours before the game and went to a Chicago hospital. A chest X-ray ruled out pneumonia. He was admitted for more testing Thursday.
Lessard said Baker did not suffer a heart attack.
The Reds are a combination of three victories, or losses by the St. Louis Cardinals, away from clinching the National League Central Division title.