He arrived in a trade for virtually nothing, an aging quarterback who the Cardinals hoped could help new coach Bruce Arians jump-start an offense that had floundered since Kurt Warner's retirement.

Carson Palmer provided much more than that.

Palmer's time as a Cardinal and in the NFL is over. The NFL's No. 1 overall pick in the 2003 draft, who turned 38 last week, announced his retirement Tuesday via a letter, closing out a 15-season career – the last five of which were spent with the Cardinals.

"Over the years, I've had teammates who decided to hang it up and I would ask them how they knew when it was time to walk away," Palmer wrote. "The answer was almost always the same: You just know.

"For me that time is now. Why? Quite simply, I just know."

Palmer's announcement comes a day after coach Bruce Arians did the same. Wide receiver Larry Fitzgerald, who is also contemplating retirement, is expected to take some time before deciding if he will play in 2018. Palmer will still cost about $6.6 million in dead cap space for 2018, but his departure is expected to save $14M on the cap.

The Cardinals, however, now do not have any quarterbacks under contract for 2018. Blaine Gabbert, Drew Stanton and Matt Barkley are all scheduled to become free agents.

Palmer's tenure didn't end the way he wanted, with a broken left arm in London that meant he played only seven games this season. But like Warner, he celebrated a career rebirth with the Cardinals, meshing into Arians' offense, becoming one of the top passers in franchise history and leading the team – during his own MVP-quality season – to the NFC Championship game in 2015.

Palmer was also a key leader in the locker room with his dry wit and his unflappable demeanor.

"It's been fantastic, some of the best (years) of his career, some of the best of my career," Arians said of Palmer's time in Arizona. "His numbers and the wins. His leadership. It's just a shame he had the knee (injury) and now the arm, or I think he'd have done wondrous things in his time here."

Palmer's transition with Arians wasn't completely smooth, with interceptions coming far too often in the first half of the 2013 season as Palmer and the wide receiving corps tried to learn a complicated system. But Palmer found his groove the second half of that season, and went 6-0 as a starter in 2014 as shoulder problems and a torn ACL eventually ended his year.

Then came his fantastic performance in 2015. Palmer threw for a career-high 4,671 yards and 35 touchdowns asthe Cardinals went 13-3 before losing to Carolina the step before the Super Bowl. While Fitzgerald's amazing run after the catch is what is remembered about the Cards' dramatic win in overtime over the Packers in the NFC divisional playoff game, it was Palmer's own play – spinning away from a sure sack before finding Fitzgerald – that was equally important on that play.

"He's been an unbelievable teammate," Fitzgerald said last week. "I've seen him grow as a player, picking up this system and having some of the best numbers of his career. I still think he has a lot of great football in him. What he's going to decide to do, I don't know. Whatever he does decide to do, we'll all respect that."

Stanton was signed as a free agent in March of 2013 with the original intent of him becoming Arians' first starting quarterback. But General Manager Steve Keim pulled off an impressive trade with the Oakland Raiders, getting Palmer and a seventh-round pick in 2013 for only a 2013 sixth-round pick and a seventh-round pick in 2014.

Palmer ended up with 38 wins as a starter, third in franchise history, despite missing 10 games in 2014 with his ACL tear and nine games this season. He is 12th in NFL history in passing yards (46,247), 12th in touchdown passes (294) and 11th in completions (3,941).

Meanwhile, Palmer and Stanton formed a bond themselves.

"Five years is an eternity for an NFL quarterback room to stay together," Stanton said. "I just think his approach, the way he approaches the game, his mentality, his preparation … I've got a lot of respect for him, probably as much as anybody that I've ever played with, and genuinely look up to him on and off the football field."

That Palmer left through a letter and not a press conference was not surprising. He already is on vacation with his family, and while he was always good with the public side of his job, he did it because it was part of the job. It was always the hidden part of the game, working behind the scenes, that Palmer relished the most.

"I'll especially miss the grind," Palmer wrote. "It's the part I don't think people fully appreciate, maybe because many NFL players make the game look so easy and effortless. The mental and physical preparation it takes to compete -- week in and week out, year in and year out -- is draining and grueling but has always been my favorite part."