Not sure you can blame the CBs for that goal (although I appreciate why people want to). The guy was allowed to break by Lemina and Romeu. Once he was through it was a good goal
You absolutely have to look at the midfielders, when an opposition midfielder breaks through unmarked. But equally, you can't ignore the massive gap between the defenders. Stephens seems to wonder off to become a make-shift RB - despite there being no-one in that area to pick up.