Arizona's historically bad O-line

John Skelton (pictured) and Kevin Kolb have been under constant pressure this season. Peter Aiken/Getty Images

The Arizona Cardinals haven't just had poor pass protection this season; they have had historically disastrous protection. And they've already had two quarterbacks taken out of games due to injuries.

Things aren't likely to get any better for the Cardinals as they host the San Francisco 49ers and one of the league's best defenses on Monday night.

Although the 49ers haven't been getting to the quarterback as frequently as they did in 2011, the Cardinals' coaches and John Skelton have to be worried about what San Francisco will do to a pass-protection unit that has been hemorrhaging pressure against every team it has faced.

Pro Football Focus has been grading every player on every play in the NFL season since 2008, and this Arizona offensive line is allowing pressure at a greater rate than any other team in that time span -- and by a good margin. On 300 passing plays this season, the Cardinals have already allowed an astounding 134 total QB disruptions (sacks, knockdowns and hurries), and Kevin Kolb and Skelton have felt heat on 43 percent of all passing plays.