"It's been aggravating me all year," Willingham said. "I don't know why or when it started. It affects my ability to run, which affects my defense. Not that I'm a Gold Glover, we all know that I'm not, but I can't be the best I can be because it hurts to run, so I wanted to get it knocked out with the medicine in there and hopefully play Tuesday."
It's the first time Willingham has received a cortisone shot, so he admitted he's not sure if he'll be limited to designated hitter duty in the short-term once he returns.
"I don't know," Willingham said. "I don't know how well the medicine will work. So we'll just have to play it by ear, day by day."
With Willingham out, rookie Oswaldo Arcia made his second straight start in left field.
Rhett Bollinger is a reporter for MLB.com. Read his blog, Bollinger Beat, and follow him on Twitter @RhettBollinger. This story was not subject to the approval of Major League Baseball or its clubs.