Dusty Baker was diagnosed with an irregular heartbeat on Wednesday night and was expected to stay overnight in a Chicago hospital.

Baker left Wrigley Field before Wednesday night's game, and a spokesman said he would have X-rays to "rule out pneumonia."

"I guess the good thing is it's not pneumonia, but the bad thing is he has an irregular heartbeat, so basically they will keep him over at the hospital and run some more tests in the morning before he is set free," Reds trainer Paul Lessard said.

Baker will not be with the team on Thursday and his return is uncertain, according to an ESPN.com report.