Cassel is a horrible player at this point in his career. All he can do is dink and dunk, he has below average arm talent... and worse, he is a boring, non-charismatic player. He's the exact opposite of the kind of player you would want to inject to give them a "jolt". If you could put a shadow on game footage of Weeden this year to where I couldn't tell who it was, I would think it was Cassel.
Weeden is also a horrible football player. But if the Cowboys are playing a bad enough team -- and if the defense would ever decide to get a turnover or any back could break a tackle somewhere and make a play -- they can win with Weeden. Saying that it can't get any worse with Cassel is extremely dangerous. Cassel has no experience with this roster and has not played very much period over the last couple of years. If his expiration date has silently passed -- like Brad Johnson's did in 2008 -- they run the risk of losing one of the few winnable games they have left.
No QB on the roster was going to win this game today throwing to Lucky Whitehead and Devin Street. Acting like today was the straw that broke the camel's back is laughable. Whatever decision is made must be made with the idea of giving the team the best shot to beat the Eagles and Bucs. It's possible that may be Cassel, but I doubt it.