He has since undergone two ACL surgeries, overcome a multi-year elbow injury and suffered nerve damage in his throwing shoulder. The devolution in Palmer's game had reached the point that he was widely regarded as a washed-up journeyman when he first arrived in Arizona.
It's the dream of most NFL quarterbacks to maintain their physical gifts long enough to take full advantage by mastering the mental aspects of the position. Palmer is doing just that, playing the best football of his career just as he's hitting his comfort zone in Bruce Arians' system.
The only player in history with 4,000-yard passing seasons for three different teams, Palmer has "been a godsend for this organization," general manager Steve Keim recently raved.
He has not only regained his status as one of the NFL's elite signal-callers; he's also in the discussion for Most Valuable Player and Comeback Player of the Year honors.
Over his last 20 games, Palmer has completed 438 of 678 passes (64.6 percent) for 5,475 yards (8.1 YPA), a 40:15 TD-to-INT ratio and a cool 100.0 passer rating. His 17-3 record is the best in the league during that span.
If you don't trust the numbers, dial up NFL Game Pass to bear witness to a 35-year-old quarterback with more arm talent, better footwork and a higher understanding of the game than he displayed in his career year of 2005.
"He is so healthy now, and he spent so much time building not only the knee but his entire body," Arians recently told Dan Pompei of Bleacher Report. "His arm is live. His legs are so strong with all the rehab he did. And he has a great understanding of what we're trying to get done."
Pompei's excellent feature also notes that Palmer trained like a maniac this past offseason -- and it shows.
Wide receiver John Brown has testified that Palmer's arm is noticeably stronger than it was a year ago. Backup Drew Stanton believes Palmer has the best feet of any NFL quarterback. Tight end Jermaine Gresham, a former teammate in Cincinnati, credits the current version of Palmer as more relaxed, more focused, more resilient and wiser. Arians labels his quarterback "the ultimate gym rat" when it comes to film study.
Palmer's meticulous commitment to a successful return from knee surgery has had a raising-all-boats effect in Glendale.
"A lot of guys, at 35 years old, they'd hurt their knee, and get back and kind of sail off into the sunset. He didn't do that," center Lyle Sendlein told NFL Media's Albert Breer this week. "He worked his tail off to be better, and that's inspired a lot of the guys. Everyone gets nicks and bruises, and his level of commitment in coming back has guys doing a little more across the board -- do a little more studying, do a little more in drills, give a little more in everything."
"He's big, he's strong, he's accurate and he can make all the throws -- and he's playing absolutely great," receiver Larry Fitzgerald told NFL Media columnist Michael Silver after Arizona's Week 3 victory over the 49ers. "Now he's with a coach who really believes in him, and Carson's completely in command."
Fitzgerald went on to tell ESPN.com's Jim Trotter that Palmer is the "best in the business" right now.
"The way he's playing," Fitzgerald added, "I'd put him right up there with Aaron Rodgers and anyone else."