This is probably something Peterson had during the year and he still ran for 2000+ years. They say the 2nd year back from an ACL injury is when the player is close to 100% and that should be a scary thought for the league.
Vikings RB Adrian Peterson underwent a surgical procedure today to repair a sports hernia. Team says "no long-term concerns."
— JennaLaineESPN (@JennaLaineESPN) February 7, 2013