I'd wait until much later (next EL squad will not be updated until start of Feb after Jan transfer window is closed).before making definite decisions.
But given what we THINK we know about Odobert and Richarlison injuries now, I'd drop one or both of them from EL squad to get a full back into EL squad (and the full back may be Spence)..... it might be a month or two after Feb before they return from injury (or not)
However we cannot expect to win EL with only 4 forwards (Solanke, Son, Johnson, Werner) possibly counting Kulusevski as well and if Lankshear and Moore continue to progress ...... so we may need to consider a new forward in January rather than a full back (maximum 2 changes to EL squad allowed in Feb) if both Richarlison and Odobert are long term injuries. And bringing in another forward probably only makes sense if Werner's loan is definitely ending in summer. .
Even if we get to EL final, I think there are a maximum of 7 games left in EL after next squad announced, so one additional full back such as Spence and the versatile CB/LB Ben Davies is probably enough
PL squad has slightly different rules and hence dynamics ....... and with domestic cups as well we have 20+ games from January to plan a squad round, but that may mean having one or two players in PL squad who are not in EL squad which is never popular nor understood .by fans
However; that is 100% behind my thinking.
I get your caution; but even in Jan it may not be conclusively clear that Richy & Odobert will be physically reliable........ Not to tempt fate, but they could both be plagued further deep into the 2nd half of the season (in Richy's case we surely can't assume otherwise anymore)...... Just use them both to keep the other lads fresh for EL duty whenever they ARE available.
As to the above point about 4 fwds............. Taking the view of 1 x CF, 2 x WFs and 1 x more primarily attacking MF in any one EL game that's:
Solanke, Son, Johnson, Werner, Kulu, Madz + Lankshire & Moore for 4 starting spots.
.........It's not as thin as it first seems. Esp. if Richy & Odobert CAN clock up some minutes domestically and alleviate some of the overall burden.