OK, there are a lot of "experts" who keep making excuses for PP21 and try to explain away what happened and who to blame other than PP21, but consider that No less than the top local defensive expert Todd Bowles keeps pointing out that PP21 needs to improve his footwork and his techniques instead of merely trying to play off of his talent level alone.
Sure, go ahead and blame the safeties for PP21's porous coverage, but all you are really pointing out is PP21 can't do any better in his defensive coverages than the next DB on the roster and his huge contract and the Cap hit are really not necessary or justified.
DBs like Revis and Talib are able to play receivers tighter and closer than PP21 through better techniques, footwork, and speed. They work to stay close and play to break up passes instead of how PP21 does by allowing receivers to generate space and catch the ball and then afterwards try to defense the receiver. If your excuse is PP21 is too big, then PP21 belongs at safety. If you recognize he no longer has his rookie year closing speed, then play him at safety.
Bottom line, Todd Bowles and Keim (real NFL experts and professionals) both indicate PP21 is not playing as an elite DB and needs to improve. Otherwise, why pay him huge and elite money when he is not producing like an elite dB.
BTW, the Broncos begin to easily have their way with the Cards defense ONLY AFTER Campbell and Shaunessy were knocked out of the game and the Broncos run attack begin to click. As the excuses had already pointed out, Manning had his way with PP21 because the safeties were remiss on their coverages, so it was more of the same for he Broncos after PP21 went out.
