He is very good to great player. One of the better slot corners in the league who plays great at the LOS against the run & is a threat to blitz the QB. Unfortunately, he was signed to big contract on the belief that he was going to continue to be a big time play-maker. While he has been pretty good over the last two seasons, he has taken a step back in coverage while not making enough game changing plays to justify his contract.
Add in a new defense minded HC along with a new DC, means there were going to be some changes. Mathieu became expendable at his former price.
I wish him the best of luck and had hoped the Cards could have worked out a deal to keep him.