Rich Gannon
There’s a strong argument to be made that Gannon was one of the most underrated quarterbacks of his generation. However, he hung on for too long with the Raiders because injuries took their toll. These restricted his appearances on the playing field and affected his performances.
His quarterback rating plummeted from a career-high 97.3 in 2002 to 73.5 the following year. It rose again the following year but this came with the caveat that he played fewer games. Gannon had a decent career but everything went wrong quickly for him (via Yardbreaker).