Practice and preseason are played at approximately 3/4 speed. A lot of guys look good or great in practice but cannot replicate in real games. RJ is a good example. He couldn't handle the pressure or rush a huge portion of the time, which was non-existent in practice and preseason.
The best example to me was Trent Edwards. He also was a practice and preseason all-star. And don't talk to me about the 4-0 or 5-1 start before he got clobbered. We won a few games but he wasn't that good and didn't show the talents he needed to show to be good regardless of raw stats.
When Chan was made coach originally, he saw how good Trent looked in practice and preseason, and thought, as good and great coaches do, that I have a lot to work with here, and I can coach him up. He started Trent for two or three games and immediately saw that TE couldn't handle the rush. All he could do, like Peterman, was read the defense pre-snap, make a quick decision and with his quick release throw an accurate short completion.
Chan didn't decide to bench him for Fitzy the backup, Chan just cut Trent. You never see that. He probably just didn't want to be tempted again. The same thing happened to McDermott with Peterman.