Contreras will undergo an MRI on Monday, manager Joe Maddon said.
"We'll find out the severity of it or the lack of severity of it," Maddon said. "We'll just see how it plays."
Contreras pulled up and grabbed the back of his leg as he exited the batter's box after hitting a drive to left-center in the seventh inning against Milwaukee. The ball was caught.
Contreras is batting .275 with 19 home runs and 57 RBI.