NFL Media Insider Ian Rapoport reported that the diagnosis was made after initial tests on the free safety's knee, according to a source. An MRI is expected to confirm the injury Monday.
Mathieu was injured in the third quarter after getting his left leg tangled while returning a kickoff. Mathieu was down on the field for a short time, then attempted to jog off the field under his own power. However, he collapsed as he neared his sideline, and he was immediately carted to the locker room.
It's a terrible setback for Mathieu, who became an instant starter for Arizona and had put himself in the running as the league's Defensive Rookie of the Year. The Cardinals' secondary will be challenged to fill the void left by one of the NFL's most promising defenders.