Power Rankings: No. 26 Minnesota Vikings

A weekly examination of the Vikings' ESPN.com Power Ranking:

Preseason: 17 | Last Week: 23 | ESPN.com Power Ranking since 2002

The Vikings continue to slip in our Power Rankings primarily because of how bad their defense has been. They have allowed the most touchdown passes in the league (nine), and they've already given up nearly 1,000 yards passing in three games. Cornerbacks Chris Cook and A.J. Jefferson, as well as safety Jamarca Sanford, could be hard-pressed to return from injuries in time to play on Sunday against the Steelers, and although Pittsburgh doesn't present an obvious threat in the passing game, neither did the Browns last week. Third-string quarterback Brian Hoyer still threw for 321 yards and three touchdowns, recovering from a pair of interceptions to become the second straight quarterback to beat the Vikings on a two-minute drill.

If the Vikings can't win in London on Sunday, there could be changes afoot after their bye week. Quarterback Christian Ponder has continued to struggle, and a week off might provide the perfect juncture for the Vikings to switch to Matt Cassel. The veteran quarterback had plenty of his own struggles in the preseason and is coming off a miserable year with the Chiefs. Moreover, a switch to Cassel would trigger all sorts of larger questions about Ponder and the decision-makers (general manager Rick Spielman and coach Leslie Frazier) who used the 12th overall pick on him in 2011. Ponder hasn't been the Vikings' only problem, and with surprising lapses on their offensive line, he might not even be their biggest concern on offense. But he's certainly high on the list, and absent a complete performance on Sunday, the Vikings might decide that lifting him is one way to give their team a spark.