I think it's fair to call him the weakest link. Look at the starting eleven.
----------------------Lloris
----Dier--Alderweireld-Vertonghen
Walker--Dembele----Wanyama---Rose
----Eriksen--------Kane----------Alli
That's a terrific team, when fully healthy it's the 6th or 7th best team in Europe , and definitely the best in England*. Dier is probably the most easily replaceable guy in that team, you could swap him out for any solid CB in England (Wimmer, for example) and the starting eleven as a whole would do more or less the same. I like Dier and I think he's going to become a beast of a DM/CB, probably like a white Mascherano, but still.
*Kane/Alderweireld/Dembele were missing for about 2 months combined, that's the difference between Spurs and Chelsea