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

Harvin hasn't been at the practice facility the past two days. He 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 victory against Arizona.

Childress said Sidney Rice (hip) practiced "very hard" Thursday. Rice would need to be activated by Saturday to be eligible for Sunday's game. Childress said Rice has experienced a few "tweaks" in his hip this week, but nothing too serious.

The Vikings could be without cornerback Asher Allen. He suffered a concussion Sunday and hasn't practiced this week. If he's not available, rookie Chris Cook would be the starter at Chicago on Sunday and either Lito Sheppard or Frank Walker would play in the nickel.

Better ball security

Adrian Peterson's fumbling drew a lot of scrutiny and attention last season, but ball security has not been an issue for him so far this season. He has not fumbled, and he is rarely asked about it.

"It's been cool," he said. "I'll just wait and see what else they'll come up with."

Peterson has been criticized for his pass blocking, but he insists he's made improvement. Peterson appeared to miss several blitz pickups against Arizona, but he disputed that number.

"It was actually one I missed on. One," he said. "From the viewers point of view, [they were] probably like, 'That was on Adrian.' That wasn't the fact. I think my pass protection is good. That doesn't have anything to do with why I'm not out on there on third downs. It's play-calling. They've got certain plays that they want called that I'm out there on third down and Toby [Gerhart] is out there on third down.""