The Detroit Lions have suffered their first injury of training camp.

Tailback Theo Riddick participated the first two days of camp, but missed Sunday's padded practice due to an issue that coach Jim Caldwell declined to disclose. He watched from the sidelines in shorts and a baseball cap.

"Anytime you don't have someone out there you'd like them to be out there working," Lions coach Jim Caldwell said. "But he'll be all right."