Vikings coach Brad Childress said Percy Harvin visited with a doctor for the second day in a row Thursday as the wide receiver continues to deal with a recurrence of migraines.

Harvin also missed practice on Wednesday, the first time since Sept. 22 that he had been forced to sit because of the debilitating headaches. It's not certain if Harvin will be able to practice on Friday. He hasn't even been at the facility the past two days.

Harvin missed the Wednesday and Thursday sessions last week because of a sprained left ankle but set career highs with nine catches for 126 yards in a 27-24 overtime victory against Arizona.

Childress said that Sidney Rice (hip) practiced "very hard" on Thursday and that the team and Rice will simply have to see how the wide receiver comes back on Friday. Rice has yet to play this season and would need to be activated by Saturday afternoon to be eligible for Sunday's game.

As for the defense, the Vikings could be without Asher Allen, who has been starting at the right cornerback spot in place of the injured Cedric Griffin. Allen suffered a concussion last Sunday and hasn't taken part in practice this week. There appears a good chance he won't be available for Sunday's game in Chicago, meaning rookie Chris Cook would be the starter at the right corner and either Lito Sheppard or Frank Walker would play in the nickel.

Safety Jamarca Sanford, a special-teams ace who suffered a hamstring injury against the Cardinals, missed a second consecutive day of practice on Thursday as well.

Quarterback Brett Favre (ankle, foot, calf) did not practice Wednesday but did limited work on Thursday. Right tackle Phil Loadholt (knee) and nose tackle Pat Williams (elbow) also were limited.

Defensive end Everson Griffen (shoulder) and cornerbacks Sheppard (hand) and Walker (hamstring) remain on the injury report but have not missed any practice time this week.

For the Bears, center Olin Kreutz sat out again Thursday because of a hamstring injury but there remains a decent chance he will play. Cornerback Zackary Bowman (foot) was limited and cornerbacks Tim Jennings (knee) and D.J. Moore (head) went from limited on Wednesday to full participation on Thursday.