"He's got a severe hamstring," Bears coach Marc Trestman said.
Hayden tore his hamstring Saturday night during a practice at Soldier Field. Trestman said Hayden plans to undergo surgery and expects a six- to eight-month recovery for the 30-year-old cornerback, who is in his ninth NFL season and second with the Bears.
The latest setback represents the second season-ending injury suffered by a member of the defense since the start of training camp. Defensive end Turk McBride was lost for the season during the first week of camp because of a torn Achilles tendon.
Hayden underwent MRIs on Tuesday and Wednesday and will undergo surgery as soon as possible, depending on results of Wednesday's MRI. The injury was so severe, Hayden experienced difficulty on Tuesday just standing up.
Hayden didn't expect to receive such a dire diagnosis, according to a source familiar with the situation. The source said Hayden's injury is similar to the torn hamstring suffered by former Bears defensive tackle Tommie Harris in December 2006. That injury also required surgery, and an extensive rehabilitation program that forced him to miss the last four regular-season games, and the club's entire postseason run.
In Hayden's absence, second-year cornerback Isaiah Frey has been taking the reps at nickelback.
"You hate to see that happen," Frey said. "I'm praying for him that he gets healthy really fast."
After bouncing back and forth between cornerback and nickelback last season, Hayden was the clear-cut No. 1 nickelback on the roster in camp. He had been working exclusively at the position since he signed a one-year deal to return to the Bears in late March.
Hayden played in all 16 games with the Bears last season after spending an injury-plagued 2011 season with the Atlanta Falcons.
Free safety Chris Conte believes Frey is capable of filling the vacancy at nickel corner, saying Frey "definitely has what it takes," but lamented the loss of a veteran member of the defense.
"Kelvin has worked so hard," Conte said. "He's been doing a great job, and it's really tough to see him go down like that. I know none of the DBs really expected it to be that serious. To hear he's out for the whole year is kind of shocking. So prayers and thoughts go to him and his family, and hopefully surgery goes well and he's back soon."
ESPNChicago.com's Jeff Dickerson contributed to this report.