EDEN PRAIRIE, Minn. -- Brett Favre's ironman streak is in jeopardy.

The 41-year-old Minnesota quarterback has a stress fracture in his left ankle that could end his NFL-record run at 291 consecutive games started -- 315 including the playoffs.

Vikings coach Brad Childress said Monday an MRI on Favre's foot revealed the stress fracture as well as an "avulsion" fracture in the heel bone. An avulsion fracture occurs when a fragment of bone is torn away by a tendon or ligament.

"He's got great pain threshold and also great competitive zeal," Childress said, succinctly summing up Favre's legacy of durability.

ESPN medical analyst Dr. Michael Kaplan, who has not examined Favre, told ESPN NFL Insider Adam Schefter that the quarterback's injury is serious.

"Every other mortal would be out, but with [Favre], it's a longshot he could play," Kaplan told Schefter.

Kaplan said Favre could inject a long-acting anesthetic in and around the ankle to help him play. It would take away the pain for the game, but it could affect Favre's throwing.

Childress said neither injury requires surgery, but Kaplan said Favre may need an operation at some point after his playing career to either fuse or replace his injured ankle.

Favre is also the subject of an NFL investigation into allegations that he sent lewd photographs and suggestive messages to a female New York Jets employee in 2008, a development that first put his streak in danger with the possibility of a suspension under the league's personal conduct policy.

NFL spokesman Greg Aiello said Monday there was nothing to report about the investigation, but league sources told Schefter that the league could end its investigation this week.

Thus far, Jenn Sterger, the woman at the center of the investigation, has not cooperated with the NFL and according to the sources, the league nearly closed the investigation after Favre spoke to NFL security in Minneapolis last week.

Fox News reported Sunday that Favre admitted to the league that he left voicemails for Sterger, but denied sending the photographs.

The NFL left the investigation open in the event that Sterger would agree to step forward and share her story with the NFL, but the league is comfortable with the people it has interviewed and the information it has gathered, the sources said.