You though that he played pretty well in 2013? He averaged 200 yards/game and threw 11 TDs vs 9 INTs - Very very close to EJ Manuel's rookie stats where he played with only 1 WR that had ever caught an NFL pass going into the season.
In Cassel's 4 years since his miracle 2010 season in KC, he has put up the following over 30 games:
Year Yards/Attempt Yds/Game TD Int Rate
2014 6.0 142 3 4 66
2013 7.1 201 11 9 81.6
2012 6.5 200 6 12 67
2011 6.4 190 10 9 77
Over those 30 games, he 30 TDs and 34 INTs - that's a good-sized and most-recent sample and it isn't very good. Heck, I'd say EJ Manuel's stats over his first 14 games are at least as good.
I didn't post this to say that Manuel is the answer, but if Cassel is still being thought of as a possible answer 10 years into his career, then Manuel certainly ought to get at least this year before he is written off.