Jackson, a fan favorite and highly respected inside the Texans' locker room, reportedly received a three-year, $33 million deal. Last season he agreed to a one-year, $7 million contract to play for Houston.
After early reports that Mathieu had turned down a $9.5 million per year offer to stay with the Houston Texans and would explore the National Football League free agency market, ESPN's Adam Schefter reported on Monday night that the Chiefs will sign the star-caliber safety.
His best season as a pro came during the 2015 season with the Arizona Cardinals when he was named an All-Pro, and went to the Pro Bowl after creating six turnovers, five were interceptions.
Mathieu will now man the back end of the Chiefs secondary with another big-name safety, Eric Berry. The Texans have made keeping him a priority this offseason.
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And he warned Tory MPs who plan to vote against the deal that it could mean Jeremy Corbyn becoming prime minister. David Davis told talkRADIO's Julia Hartley-Brewer: "It all hinges on what Geoffrey Cox says in his legal advice".
Mathieu, 26, was selected by the Cardinals in the third-round of the 2013 draft.
Instead, Mathieu will earn an average of $14 million per year - the same amount freed up by Justin Houston's release - in Kansas City as a key piece of new coordinator Steve Spagnuolo's defense.
Mathieu can't officially sign until the league year begins on Wednesday, but he can negotiate with teams.
The NFL Network's Ian Rappoport is also reporting about the impeding free agent deal, saying Mathieu was their top target. He only played seven games in 2017 as he battled an ongoing foot injury, and he finished that season on injured reserve.