Dilfer pointed to what he calls "critical situations" for quarterbacks, including third down, and suggested Shanahan might be discovering McNabb's relative shortcomings in these areas.
The point resonated following multiple offseason discussions about whether an NFC West team, notably the San Francisco 49ers, should have pursued McNabb during the offseason. I've broken out data from an ESPN Stats & Information packet showing third-down stats for current and recent NFC West quarterbacks, plus McNabb.
The 49ers' Troy Smith has completed 2 of 5 passes on third down for 30 yards and a 70.4 rating. He's not on the chart because the information covers only quarterbacks with at least 12 third-down attempts this season.
This Sam Bradford guy might turn out to be OK.