The Pirates had already placed catcher Francisco Cervelli on the 15-day disabled list earlier Saturday after having surgery to repair a broken hamate bone.
So, Kratz was up next. He caught the final two innings of the Pirates' 5-1 loss behind pitchers Rob Scahill and Arquimedes Caminero.
"You just try to get your feet under you," Kratz said. "And it's baseball. I don't think I've caught anybody like Caminero before, but it was exciting to be out there and rip the Band-Aid off and get ready to go."
Stewart will be available as a backup in the final game of the series, but Kratz got the start. Kratz has met with Pirates manager Clint Hurdle, pitching coach Ray Searage and bullpen coach Euclides Rojas to further discuss the Pirates' pitchers and their tendencies.
Even though Kratz has been with the team for roughly a day, Hurdle has confidence in his abilities. Kratz has played in 208 Major League games with the Pirates, Phillies, Blue Jays, Royals and Astros since 2010. He was signed by the Angels as a Minor League free agent on May 27 and was sent to Triple-A in Salt Lake City.
"It's not easy, but it's what he does," Hurdle said. "It's kind of like a wood carver going to somebody else's house, they've got the tools, they've got the wood. He just goes to work. He knows what he's doing."
Kratz's biggest goal is to get Pittsburgh's pitchers to trust him from the get-go.
"It's them looking in there and trusting I'm there for them and can help them in whatever way," Kratz said.
Sarah K. Spencer is a reporter for MLB.com based in Pittsburgh. This story was not subject to the approval of Major League Baseball or its clubs.