I think the biggest need we have at the moment in terms of squad depth/back-up is in the midfield.
The best MF option we have is Lo Celso-PEH-Ndombele. That leaves us with zero dynamic/creative/attacking options off the bench in MF. We also have zero replacement for Lo Celso or Ndombele if one is injured or needs a rest so we become so reliant on the one healthy guy. Bringing in a (young) creative MF should be a priority. Moura and Alli are the two guys I would move out to bring in money to fund the move and create space in squad for this type of player. Both guys are not needed and offer little to the team at this point.
Also, there is zero back-up for PEH meaning that he has been overplayed across all competitions and is needed to stay healthy and in top form. Skipp seems like a long term solution but should stay in Norwich where is able to have the ups and downs needed to properly develop. I think this is a spot we should look to in the January window and maybe bring in a vet on an expiring deal that we can get cheap but can provide some cover in that spot, as well as be brought on to see out games instead of guys like Winks, Sissoko and Moura.
CB is obviously a major issue but for me this is an issue at the starting level not bench. If we brought in two new starters a bench of Toby, Tanganga, Rodon and Ben as an emergency option is really good. Obviously that list does not include Dier or Sanchez who should be moved out with the two new players (ideally) brought in.
Hart is a terrible option as a back-up keeper but hopefully Lloris stays healthy so that it doesn't really come into to play. With Europa he is fine because we can dominate these teams enough that he doesn't come into to play but if we are in CL next year he cannot be our option for starting group games as he is not close to good enough and will be tested in those games.