Think the basics is what he struggles with most of all though, which is why he's such a worry.
. . .
This whole post was an excellent summary. The first and foremost duty of a central defender is to stop opponents from scoring goals, any other qualities are a bonus. In practice this means being in the right place at the right time (positioning), and then being able to ensure opponent does not get dangerous shots and headers towards the goal (defensive skills).
Sakho's positioning is often puzzlingly weak. That may be connected to our general defensive organisational problems, but it is curious how, for example, Kolo Toure (who plays at least as sporadically) does not have similar issues. This alone would be a concern.
Worse, albeit Sakho indeed seems to have the physical qualities to be a world-class duel-winner, he frequently fails to make them count, letting opponents get quality headers and shots towards the goal time and time again despite being right next to them.
Like others have already pointed out, Lovren had several glaring problems in his game as well, and he has improved dramatically lately, so I'm not giving up hope for Sakho either just yet. But I'm seriously starting to fear there is more to Sakho's struggling than just a patch of bad form.