It's a foot injury. More from Ken Gurnick:
Kershaw took a painful step going to first base Monday night, and Mattingly said it's "a little plantar fasciitis. He took a funny step and just felt it."
Mattingly said Kershaw has been receiving treatment for the condition for "at least a week."
"He says he doesn't feel it at all [while pitching, only when running]. They're trying different tape and things at night to help it go away," Mattingly said.
The wisecrack response to feeling it while running is "don't run", although that's not usually an option for a baseball player. Plantar fasciitis can linger, so it's good the Dodgers found it quickly. It didn't seem to bother Kershaw Monday night, when he threw seven solid innings against the Phillies.