Technically speaking, the reason Chad Henne had to have shoulder surgery was because Lex Hilliard forgot to take the handoff, so it was a busted play, so Chad Henne ran for a 1 yd gain, and then injured his shoulder when he got mauled by Shaun Phillips.
That play is what, IMO, sealed Hilliard's fate. It was such a complete misstep by him, and while he had made small errors here and there (especially on blocking assignments), a large error was compounded by the injury of the team's starting QB