Mathieu, 25, had a fine season in 2015 when he recorded 89 total tackles, five interceptions and 17 pass breakups to earn a Pro Bowl selection.
He had signed a five-year, $62.5 million deal with $40 million guaranteed in the summer of 2016.
The Cardinals could have spread the cap hit over the next two years, with $3.1 million applying this year and $6.2 million next year.
The Cardinals and Mathieu were involved in contract talks leading up to his release. Many teams - including the Texans, Giants, Jets and Steelers are rumored to be hot after Mathieu's services and the Pats would likely have to win a bidding war to land him. The 25-year-old safety mentioned that he was not willing to take a pay cut to National Football League reporter Gregg Rosenthal.
"There's many ways you could restructure a contract instead of just taking money out of my pocket", Mathieu said at the time. The Patriots also could use a young, versatile safety. First, Melvin Ingram shot out a tweet, and then Melvin Gordon tagged Mathieu in a tweet, just to make sure he's aware that the Chargers want him. His presence would make the need for safety and linebacker less being able to play either position. While the Pro Bowler won't come cheap, he would add immediate impact to both the run and passing game for the Bengals defense.
Despite that production, the Cardinals reportedly asked Mathieu to take a pay cut from his current deal, which was set to pay a $13.75 million salary for 2018 along with a $5 million roster bonus Friday. He had a reputation for having a nose for the football and played with a reckless abandon that helped shape who he was as a safety. Whatever the numbers, there surely will be a high level of interest in his services.