Rookie Hercules Mata'afa was just settling back into the defensive line when he tore his anterior cruciate ligament Wednesday, a league source confirmed to the Star Tribune.

Mata'afa, the former Washington State star, signed with the Vikings as an undrafted free agent last month. The Vikings originally tried Mata'afa, listed 6-foot-2 and 254 pounds, at linebacker. But the role only lasted a couple practices, Mata'afa said Tuesday.

The Vikings moved him to defensive end after rookie minicamp.

"With the big dogs," Mata'afa said. "It feels like home. It's way easier for me to understand the concepts. It's just what I've done all my life, so I feel comfortable doing it."

He's the latest NFL player to suffer a season-ending knee injury in voluntary workouts this spring. The Eagles reportedly lost linebacker Paul Worrilow to a torn ACL, just as the Rams lost defensive end Morgan Fox and Chargers tight end Hunter Henry.

DraftAnalyst.com first reported Mata'afa's injury.