He wasn't anything special during the last season. He put in some solid performances but was still nowhere near the level of Pique or even Mascherano. His goals scored don't give merit to his quality as a defender any more than Bartra's do. There's a reason nobody who talks about him chasing Bale also mentions him scoring the equalizer in that very half.
Still, he was our 3rd best Cb and our only option when we needed depth.
Bartra is Bartra and we can't seriously count on him.
Vermaelen was injured the whole Season.
Mathieu helped a lot last Season in decisive CL matches and in the last 10-15 rounds of La liga.
He has flaws, but even now, he is our 3rd best Cb.