Yes, Carson Palmer is a two-time Pro Bowler and a former Heisman winner. But he's simply not as good as he was five years ago.
Since his solid 2005 season, Palmer's TD throws and completion percentage have decreased while his interceptions have increased. Not a good trend to have.
Still, he's in the upper half of starting QBs in the league, so why worry about replacing him? One word: injuries.
You probably recall the nasty hit Palmer took in the playoffs in 2005 that caused him to have a potential career-ending knee injury. After he came back from surgery and rehab from that, he played extremely well.
In 2008, however, Palmer suffered from a torn ligament in his elbow. Rather than electing to have surgery, Palmer went through rehab and rest to heal it.
This season, Palmer had issues with a sprained thumb. One has to wonder if concerns over his durability could cause the Bengals' front office to look for a solid backup. Both Palmer and his current understudy, J.T. O'Sullivan, are 30 years old, so age will also become a factor at some point.