The game-time temperature was 84 degrees at Wrigley Field, and it was humid. A few fans in the crowd of 39,619 had to be treated, as well, for heat-related problems.
The Cubs' athletic trainers attended to Nauert between innings throughout the game with wet towels and provided water and Gatorade. First-base umpire Doug Eddings took over for Nauert behind the plate.
"It gets hot here, it gets humid here," Cubs manager Mike Quade said. "I hope he's OK."
A spokesman for Major League Baseball said Nauert was day to day.
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter@CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.