The lower leg weakness that hindered Vikings receiver Jerome Simpson during Sunday's 30-7 victory over Tennessee was caused by a back problem, according to an NFL source.
Simpson underwent a magnetic resonance imaging exam Monday in an attempt to diagnose the strange weakness he felt in his leg.
The MRI revealed a back issue, the specifics and severity of which are not yet fully known.
Simpson will undergo further testing and visit with additional doctors Tuesday. But at present, according to the source, Simpson's injury isn't thought to be serious.
The speedy receiver was bothered throughout the Sunday's game by the ailment, which apparently sprung up out of the blue.
Vikings coach Leslie Frazier said Simpson woke up Sunday with numbness and weakness in his leg and reported having difficulty pushing off as he went through some of his pre-game warm-up exercises.
Most troubling to Simpson was the fact that he had never experienced anything similar.
"This was completely new to him," Frazier said. "So as you can imagine it scared him and had him alarmed."
Simpson practiced full-out all last week and was never even on the injury report.
In fact, against a weak Tennessee pass defense, Frazier said the Vikings "had a big package for Jerome," expecting him to be healthy and play a big role.
But Simpson told Frazier and head athletic trainer Eric Sugarman before Sunday's game that his leg wasn't feeling right. Frazier said team doctors didn't see any harm in clearing Simpson to play.
Simpson dropped the first pass thrown his way on the first drive and came back to the sideline frustrated.
He wound up playing only 24 of the team's 69 offensive snaps and didn't have a catch, consistently bothered by the weakness and numbness in his leg.
"We were hoping as time went on he would regain that feeling during the course of the game," Frazier said. "In talking to the doctors they felt like that was possible. And it did get a little bit better but not to the point where he felt completely comfortable."