People have identified Johnson as a poor defender, who switches off repeatedly, for years. There are reasons for thinking this, as described by PoP/BoH. Still, he gets picked to start by every manager, Liverpool and England (is that eight of them?) since he came to Liverpool. We have also seen how much worse we seem to be when he doesn't play. Finally, for all his perceived defensive weaknesses (and several other players on the team) there are only three teams who have given up fewer goals than us this year.
It seems to me that Johnson is one of those players that seems not to do much right, but the team he plays for seems to win more with him than without him. In ice hockey there is a statistic called plus/minus which is simply goal difference when you are playing, with unlimited substitutions from a squad of twenty players play about twenty minutes out of sixty a game. The players near the top of these lists aren't necessarily the "best players" and sometimes seem quite baffling. Perhaps Johnson does things that don't seem important, have no stat, but are actually important.
I wonder what it is about Johnson that might make him a better player than he appears? Is he the sort of player to whiom the renowned difficulty with football stats applies? Does his attacking threat simply negate most attacks down that wing? Are his mistakes glaring but infrequent? Does the defensive ability of full backs not matter much any more?