As the league investigates whether the concussion protocol was properly followed with Carolina Panthers quarterback Cam Newton on Sunday in New Orleans, the NFL now has an unexpected factor to consider.

In addition to getting poked in the eye as the team said he was, Newton also injured his knee during the Panthers' 31-26 wild-card loss to the Saints. The NFL learned about the injury while interviewing Panthers officials over the phone this week.

A league source told ESPN that the bothersome knee injury was a contributing factor when Newton staggered to the ground and struggled to stay balanced once he took a knee after a hard hit to the head from Saints defensive tackle David Onyemata.

Cam Newton was examined on the sideline after being brought down by Saints defensive tackle David Onyemata, but he was not taken to the locker room per new concussion protocol guidelines. Chuck Cook/USA TODAY Sports

Newton underwent an MRI for the knee injury Monday, which came back positive but is not considered serious, according to a source. A source also told ESPN that Newton would have been ready to play had the Panthers advanced to this weekend's NFC divisional playoff game.

The Panthers said Newton was poked in the eye and evaluated for a concussion. However, Newton was taken only to the team's blue medical tent, not the locker room as new NFL rules mandate.

The league's new concussion protocol, amended recently, states: "A player who exhibits gross motor instability or significant loss of balance must be taken to the locker room for evaluation if it is not diagnosed as an orthopedic injury."