"There wasn't one thing I did that hurt it," Rizzo said. "It just grabbed me. I'm not too worried about it. It's just frustrating."
Rizzo was able to loosen up on Wednesday, and he received treatment most of the day. He woke up Thursday feeling better.
"Hopefully, I can get it loose and keep it loose for a while," Rizzo said.
Rizzo said his back bothers him when he extends it, which is what he does on his swings.
"Obviously, going through a swinging motion grabs at it, too," Rizzo said. "Hopefully, today it gets a lot better. I don't think it's going to get any worse. I'll just gut through it, worst case."
Rizzo has been taking anti-inflammatory medication and keeping the team masseuse busy.
"No one is 100 percent healthy right now," Rizzo said.
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.