This a great article from the KC Star about Cassel's tenure here. You can read it for yourself, but I wanted to pull out one quote (thanks to @ChrisWesseling on Twitter for pointing this out):
"Shortly after making the trade that would come to define his time as general manager of the Chiefs, Scott Pioli began telling friends he wasn’t sure how good Matt Cassel would be as a starting NFL quarterback.
“But I know he’ll work like a great one,” Pioli said.
In that way, Cassel has lived up to his side of the deal. He has been promoted and paid like good starter, and it turns out he has the ability of a good backup. The gap is not because of a lack of effort, or caring. That’s not his fault. Neither is his tie to Pioli."
Read more here: Sam Mellinger | Cassel sets example despite abuse - KansasCity.com
Sam Mellinger | Cassel sets example despite abuse - KansasCity.com