Sources tell ESPN's Adam Schefter that Peyton Manning has been playing through a torn right quad -- specifically the rectus femoris -- for the last month.
Manning sustained the injury in Week 15 and the bruising reportedly spread to the back of his leg. In the four games since, Peyton threw a paltry four touchdowns and was intercepted four times. In 13 previous games, he threw 36 touchdowns against 11 interceptions. An inability to drive off his plant leg may have contributed to the drastic downturn in effectiveness and arm strength. However, Manning was practicing in full and it's worth noting that a "strain" is by definition a tear.
Seems like that is where the confusion comes from. He was definitely a little F'd up though.