"I knew it was going to happen," he said. "It was just, the timing caught me by surprise."
When Kerns announced earlier in the day that she was quitting the campaign, Donnelly said, "That's news to me."
In an interview Wednesday night, the Twin Peaks assemblyman said, "I was already looking for somebody else. It just wasn't a good fit there, for either of us."
He called her departure a "mutual" decision, saying, "I don't know that my brand of grassroots suited her expectations of what a gubernatorial campaign would be like."
Donnelly said, "We don't ever stop. We don't sleep ... Everybody's working their fingers to the bone."
When he promoted her to campaign manager earlier this year, Donnelly released an online video celebrating her promotion and giving her high praise.
"They say beside every good leader is a great woman," Donnelly tells the camera after opening clips of Kerns running and Donnelly drinking coffee with his wife, Rowena. "I've got two."
He said Wednesday, "You go out and make a video like we did, and it makes it tougher to make the change you need to make."
PHOTO: Republican gubernatorial candidate Tim Donnelly visits the Outdoor Sportsman store in Stockton on Feb. 11, 2014. The Sacramento Bee/David Siders