Yes, we know he straight up sucked in Detmer's offense. Taysom Hill also looked very lackluster. For that matter, every one of our QB's looked below average. It's obviously a very debated point, but it seems like Detmer's offense was too complicated / difficult for our QB's to grasp. Our QB's ALL were missing wide open receivers...and I can't help but think part of the reason is because the offense was too complicated and they had way too much multi tasking going on, so they missed open guys.
Go back to Tanner in his Freshman year and he wasn't perfect, but he was MUCH better. He was at least hitting his open guys and extending plays by rolling out, buying time for his receivers. It was a much simpler offense, which is why he was able to step in as a Freshman and be an adequate QB.
Long story short: Grimes offense should be much simpler than Detmer's, which I think bodes well for Tanner (and all our QB's for that matter). For that reason, I wouldnt be surprised at all to see Tanner win the starting QB spot come Fall.