Minnesota's Percy Harvin missed practice for a second straight day Thursday because of a migraine headache.
Coach Brad Childress said the wide receiver and kickoff returner was visiting a doctor again after his absence Wednesday.
"Just under the weather," Childress said, adding: "Whatever you do to make it through these things."
Harvin also is dealing with a sprained left ankle he suffered Oct. 31 at New England and played through last Sunday against Arizona.
After being held out of Wednesday's practice, quarterback Brett Favre was participating on a limited basis Thursday. Favre said taking Wednesday off might have helped rejuvenate his body a little as he fights ankle, foot and calf problems.
Wide receiver Sidney Rice could play Sunday at Chicago, but Childress has been noncommittal. Rice, trying to complete his comeback from hip surgery, went through a hard practice Thursday, experiencing "a few tweaks" and normal soreness, according to the coach.
Childress said he wanted to see how Rice's body responded Friday, and the Vikings have until Saturday to decide whether to activate him for the game from the physically unable-to-perform list.
Notes: DB Asher Allen has not been practicing because of a concussion. Rookie Chris Cook would start in his place Sunday if Allen can't play, and Childress said Allen has been struggling to recover this week. ... DB Jamarca Sanford (hamstring) did not practice.
The Associated Press contributed to this report.