While nobody expected the Patriots to pull off back-to-back 16-0 seasons during the regular season, it shocked me even before New England wisely dealt Cassel to Kansas City that he was considered a hot commodity with the likely return of Tom Brady.
From my vantage point, it seemed several teams had lost their mind. When I heard the Broncos had been interested in Cassel prior to the trade, I lamented at the time he did not end up in Denver.
A 10-5 record as a starter would normally be considered an impressive display. I watched several of the games that year and noticed the extreme quality difference at the quarterback position for the Patriots. A close analysis of the New England Patriots 2008 season shows a fairly easy schedule for a division winner from the previous year, yet Cassel lost five more games than the team under Brady.
One key factor Brady had which Cassel lacked, was the ability to lead a comeback against good teams. Going through each game of the 2008 season for the Patriots, there were two wins in which they won after trailing in the fourth quarter: the 4-12 Seattle Seahawks and the 2-14 Saint Louis Rams. They trailed by three early in the fourth quarter against the Seahawks and by eight against the Rams going into the final period of play.
In 2008, the Patriots lost to several subpar teams and barely beat others. With the only key player change being Cassel playing quarterback over Brady, it shows the 2008 season to be a major dropoff in quality for the team with Cassel behind center.