I believe that if we don't see significant improvement through game 6 of the season, Sitake and Detmer should consider playing Mangum. Hill has struggled, but I think he's earned the right to play through some of those struggles.
In terms of Hill's play so far this year, two things concern me:
1) While Hill's top end speed is getting close to where it was, it still seems his acceleration is not quite as explosive. So far he seems to have had a hard time getting the separation he used to when defenders close on him in the pocket, but when he's had a few yards to get up to speed, it's been the Taysom of old.
2) I'm less concerned with Taysom's long throws as I am with his intermediate throws to the second level between linebackers and safeties. I know a lot of people didn't like the Vanquish the Foe article calling for Mangum, but the passing chart in that article confirmed what I already knew--Taysom has a hard time hitting receivers 15 yards or so down the center of the field. I don't know if it's because he doesn't see that area of the field well or has a hard time reading the coverage, but middle linebackers will continue to plug run gaps until Hill can find a way to throw behind the backers (not in front or to the side of them). Additionally, effectively placing throws in front of the safety will force the safety into pay more attention to routes in front of him, hopefully allowing a receiver to sneak behind for a throw over the top.
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As a player, Detmer was very good at hitting intermediate crossing patterns--he averaged almost 10 yards per attempt over his BYU career. Hopefully, the Detmer and the rest of the coaching staff can help Hill to make this correction--as the West Coast offense is often predicated on threatening various levels of coverage.