The Cubs were doing fly-ball drills when the accident happened in shallow right field.
"They were both calling for [the ball], and it sounds like Junior might have gotten him with the back side," Renteria said. "You're talking about two strong men."
Lake converted to outfield late last season, having spent most of his career at third base.
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.