CHICAGO -- Cincinnati Reds manager 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 in street clothes more than two hours before Wednesday night's game, and a team spokesman said he would have chest 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 head trainer Paul Lessard said.

Baker will not be with the team on Thursday and his return is uncertain.

"He is not going to be here for tomorrow's game because they will be running tests. He said he will meet all the reporters in Cincinnati," Lessard said after the Reds' 6-5 victory over the Cubs in 11 innings.

Cincinnati lowered its magic number to clinch a playoff spot to one with the Los Angeles Dodgers splitting a doubleheader at Washington. The Reds' magic number for a second NL Central title in three years is three.

Lessard said Baker was having trouble breathing and has a history of having an irregular heartbeat.