Not sure a CB is essential as under Conte we have conceded very few goals - but maybe an upgrade on Sanchez and sell Rodon who is clearly not favoured - that gives us 4 x CB's in Romero, Sanchex or upgrade, Dier and Tanganga.
Defensively we could do with an upgrade on Doherty at RWB.
In centre midfield we could do with a top creative CM to compliment Skipp and Hojbjerg - with both Winks and Dele looking rejuvenated under Conte, putting a question mark over Ndombele and Lo Celso neither of whom seem to have put the efort in under Conte ?
Up front I think we need a reinforcement for Kane/Son/Moura/Bergwijn as we are one injury away from a goalscoring problem - not sure Gil is experienced enough to join that list - ideally a striker but any decent player, including a Bale on loan.
How many of those are feasible in January is another matter - and its more about player availability and selling players to create space than anything else. Good news is we have a fairly full squad - but ur issue is too many are 'average' players who we want/need to upgrade on.