The ankle has hampered Peterson throughout the season, but missing practices early in the week is likely just a way to minimize wear and tear before Sunday kickoff. There is no real reason to subject to Peterson to physical contact prior to game day.
He did return to practice on Friday, but this is an injury that will continue to bother him for awhile. Peterson said so himself, per Mark Sessler of NFL.com:
Since I twisted it up and it happened, it's been hurting consistently. Then we go out and play (against the Washington Redskins), it re-aggravates it, just the cutting and different things you're doing out there. So, it's sore, but you've just got to push through it.
Peterson will not be able to test the waters against an easy run defense this week, though, as the Cardinals have allowed only three rushing touchdowns all season. Arizona does give up over 113 yards per game on the ground, but AP's struggles have stemmed from not finding the end zone.
After two rushing scores in Week 1, Peterson has yet to return to the land of six points. However, his ankle has not stopped him from being productive. AP has averaged at least 4.6 yards per carry in each of his last three starts.
It is clear that the ankle is bothered and could be re-aggravated at any point. But AP is healthy enough to play against Arizona and can always be counted on when he is on the lineup.