It's not about Tressel. It's about Pryor. Last year he hit those passes with a lot more consistency. This year his overall abilities as a passer have improved, but his accuracy downfield has taken a step back. Pryor was more of the focal point of the offense early in the season. His struggles held back the offense as much as conservative playcalling, as seen by the dramatic shift in efficiency when they shifted back to a run-heavy attack (finally with some true read-option looks, mostly against UM).
Last year, Pryor was more of a boom or bust passer. He made some great throws downfield and could dazzle you with his theatrics in the backfield, but he would also take coverage sacks trying too hard to make the big play. In the last month or so, he has improved a lot at taking what the defense gives him, settling for a 5 yard scamper (last year he would pass on 15 yd scrambles just to try and make the pass instead) or throwing it away.
Posey & Sanzenbacher are both guys who can hurt Oregon downfield. The suspended Carter & Small were very talented but inconsistent players at wideout. Their replacements are sizzle without any steak so far. Taurian Washington is a great practice player who struggles to translate that to the field. Flash Thomas is an athlete playing WR with great natural athleticism but is still very raw at the position.