I won't say PP is not "good" but his pointed exclamation was that Sherman "is not a shutdown corner" by definition of the system he plays in.
I counter that PP is not a shutdown corner by definition of the success of his opponents. He doesn't shut anyone down.
Only 8 QBs finished last season with a 96 or better rating. PP faced 3 of them (Foles, Brees, Wilson twice). So in 12 games, 75% of last season, he was challenged with a QB who would finish the season below 96. Yet, collectively, all QBs achieved a 96 rating when throwing at PP. They were
more efficient and successful when throwing at Peterson than they were the rest of the season.
According to Pro Football Focus’s play-by-play grading system, Sherman has finished no lower than sixth among cornerbacks over the past two years; meanwhile, Peterson has finished no higher than 16th.
More stats here.
PP is riding the lingering excitement over his athleticism from the 2011 draft and his punt returns his rookie year. His coverage skills have not been close to elite. I'll keep it to the "shutdown corner" topic, but his returning averages have gone down, and when the opponent runs the ball, PP becomes Passive Peterson, so those don't really factor into any "elite player" conversation.
Peterson's statement that his job is more challenging serves more as an excuse for his average stats than a reason to be considered more valuable than Sherman.