We know that Percy Harvin missed practice Thursday because of a headache -- that is how Brad Childress described it -- and we know that when Harvin has headaches that means he is almost certainly dealing with migraines.

What we don't know is just how bad Thursday's episode might have been or if Harvin will be able to take the field Friday. Before anyone panics, remember that Harvin missed other mid-week practices during the season and was able to play by Sunday.

Sometimes he played very well in those games. One such instance occurred in Week 3 when Harvin missed time leading up to the Vikings' victory over San Francisco. He finished that game with four catches for 51 yards and returned a kickoff 101 yards for a touchdown.

Harvin, of course, missed the Vikings' victory over Cincinnati on Dec. 13 after his worst bout of the season with migraines. That resulted in a trip to the Mayo Clinic in Rochester, Minn., and until this week as far as we know he hadn't had any recurrences. Harvin also recently expressed confidence in the treatment plan devised by the doctors at the Mayo, although he declined to share any specifics.

In a week where nothing is too small to break down, we can point out that Harvin missing the Thursday practice is far from ideal. The most important (and longest) practices of the week occur on Wednesday and Thursday. While the Vikings would certainly like to see guys such as defensive ends Kevin Williams and Ray Edwards be able to take part, the fact they are sitting out because of knee injuries isn't as big of deal as Harvin having to sit.

The reason is because the Vikings do so many different things with Harvin and only Harvin. Thus, he's the guy who runs through those plays, although one would assume that Darius Reynaud could be a stand in at times. For instance, anyone who watched last Sunday's game saw Harvin line up all over the field and come in motion in different packages.

The Vikings run through all of those plays in practice and having the guy who is going to be one of the focuses of those plays on the field certainly would help. This is not to say that Harvin's absence on Thursday is going to make or break the Vikings in the NFC title game. That would be a foolish statement.

But it is a bigger deal when Harvin misses practice time than most.