I'll show my ignorance here. Is he really good? I can't remember seeing him play. I did Google him and came up with this on Wiki. It said he was an undrafted college player signed by the Cardinals:
Arizona Cardinals He was signed by the Arizona Cardinals on May 13, 2013. In just his second preseason game, against the Dallas Cowboys, Jefferson recorded 2 interceptions to go along with 5 tackles.
After the 2013 season, Jefferson became a nominal starter and racked up 74 tackles in his first year in the main line-up. He has since solidified a role on the team in the form of the third-safety, playing a large number of snaps and occasionally taking the field in end-of-game situations.
Jefferson suffered an MCL sprain in Week 16 of the 2016 season and was placed on injured reserve.[5]
He's excellent and he'd be the second best safety on the market behind Berry.