Combining Defensive Effectiveness measures is very useful for assessing the defensive strengths of various areas of the squad.
The below is a plotting of Defensive Action Success Rate % and Possession Win %. Neither include passing. Full definitions are available in the Glossary.
As at the Winter break, here is how they are doing.

I would draw the following to your attention:
Benkovic and Boyata are the most effective central defenders.
Ajer and Simunovic are not far behind.
Hendry is way off the other four.
Gamboa is the Hendry of the full backs.
Ralston has posted the best defensive numbers of all the full backs and this season has just overtaken Lustig.
Tierney's numbers are low because of the amount of time he spends attacking domestically. That means he is on the receiving end of more tackles that he gives. It is an anomaly.
Kouassi and Mulumbu are nowhere near Brown.
Neither is McGregor but this is all matches and if you plotted McGregor as “6” they would be closer.
Christie may be bottom left but his numbers are good for an attacking player.
Ntcham is not very effective at defending.
Burke, Bayo, Weah – all good fun and there are issues in those forward areas, but the central defence and full back areas (cover for left and first pick for right) remain live converns.
I’m frankly sick of writing about central defence. You can find many articles on the site. Benkovic is now injured and away at the end of the season. Boyata is of unknown state regarding his desires to stay or not even to the end of January. Simunovic is injured regularly and Ajer remains of great potential but needs senior support.
The numbers haven’t changed much and neither have the requirements.