I think he had the appendectomy before the season started and only missed the Steelers opener.
If you want to get even more precise, apparently Rothlisberger missed 4 days. In Cassel's case, 4 days would include Sunday if he had surgery on Wednesday.
That seems crazy fast to me, but I guess that's why they make the Big Money.
To me, it would be crazy to start him Sunday and risk further injury or setback his recovery time. Yeah it's a big game, but it's not the end of the world if the Chiefs lose. It's not like they clinch the division by winning Sunday. It would be helpful to win, but it's not "do or die" time.