Castillo came out of Sunday's game in Milwaukee after four innings because of soreness in his side. The Cubs had hoped that a day off on Monday would help, but he still had pain and underwent the MRI.
"I took a swing, my first swing, and I just felt something pop," Castillo said Tuesday. "I tried to keep playing, but it was getting worse and worse."
Castillo was batting .242 in 43 games with five home runs and 21 RBIs. Whiteside was a non-roster invitee this spring, and he was batting .171 in 33 games at Iowa. John Baker has been matched up with Jason Hammel and will likely get most of the work behind the plate.
"We'll continue to match up," Cubs manager Rick Renteria said. "We'll see as we move forward how it works out. Bake has done some steady catching before, and I think he's done a nice job doing the duty he has with Hammel, and he's caught [Edwin] Jackson. We'll mix and match and see what we can do moving forward."
How long will Castillo need?
"It's a situation with the rib that doesn't seem to be long term," Renteria said. "We're hopeful the inflammation surrounding it can go down. Hopefully, it's not a long stint on the DL."
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.