The Carolina Panthers have not had a consistent force at left tackle since Jordan Gross retired. Unfortunately for Cam Newton, that has resulted in him spending much of his career having to watch his own back. The best the Panthers have had at that position in the post Gross era was Michael Oher, who unfortunately only got one full season in before succumbing to severe post-concussion syndrome, ending his career.
I can say two good things about Matt Kalil this season:
That is pretty much it for the unquestioned positives. The rest of the things I am going to say are either fully negative or positive with some qualifications.