His defensive game improved drastically in the last 18 months......
Think your assessment is extremely harsh infact.....He has his weaknesses at fullback (not surprising) and the main one being a lack of pace, but his positioning, tackling use of the ball especially in tight areas and incisive passing is excellent. He is comfortable under pressure with the ball as well and despite not being a left back he is a much better option than Mendy (even if he were available).
He has played a huge part in the Prem wins we have over the last few years and you dont do that playing out of position in a Pep team if you arent a hell of a footballer.