Chelsea's one key weakness and how they can solve it in the January transfer window

The January transfer window is just around the corner and no doubt Antonio Conte will be looking to bolster his Chelsea squad for the final push of the season.
The Blues brought in five players over the summer, however the Italian spoke several times about the depth of his squad and there is one area in particular that feels that it needs more protection.
Marcos Alonso has so far started every Premier League and Champions League match for the Blues in the left wing-back role and the 26-year-old started the campaign in blistering form, especially the brace against Tottenham Hotspur.
However, his performances have faded somewhat in recent weeks and you can't help but feel that the minutes are already creeping up on a player that is asked to play in such a demanding role.
There is no back-up to his position, with Kenedy not fancied to step in when needed and no other left-footed option that might be able to even push the Spaniard for the starting spot.
Alonso still has a lot of good things to bring to the team, however he needs someone to come in to either rotate, push him to the next level or even just allow him to have a breather from time to time with such a busy schedule.
The Blues must look to that position in the winter, but who could they possibly bring in?