Just looking at the PFF grades from the Kansas game and the pass blocking jumped out at me. A middle of the road starter grade is about 63 PFF grade. BYU o-line had the following grades for pass blocking:

Lauaho - 88
Jatta - 74
Leausa - 88
Pay - 85
Etienne - 7

I've never seen a PFF score so low before in any category. If you look back at the game Retzlaff was pressured a decent amount that made him get rid of the ball early, most coming from his blind side so this grade makes sense. If you look back the last three games Etienne has regressed significantly. The first 6 games Etienne had pass blocking grades in the 70s and 80s among the leaders in B12. Then OSU and UCF it was in the 50s and then Utah was in the 40s. Then an abysmal 7 against Kansas.

I wonder if he has an injury of some kind that is causing his bad play and BYU doesn't have an alternative to replace him? What are alternative reasons that would cause such a drop off from the first 6 games?

Retzlaff needs time in order to make the reads and throws necessary for the offense to be successful. When he's under pressure he becomes squirrelly and makes critical mistakes. I would bet ASU will bring pressure on Etienne's side this upcoming game just like the last few teams have, I'm hoping they can get Etienne back to form for this game because I believe it'll be a major factor in the outcome.