We must understand that Barcelona isn't in a good condition of their financial stability. So, it makes sense if they won't plan to buy new players in January transfer window. Actually, Barcelona has some good midfielders, besides Gavi they have Frenkie de Jong, İlkay Gündoğan, Pedri, Oriol Romeu, and Fermín López. I think the number of Barcelona midfielders isn't very small, it is enough for the whole of the season. Moreover, Frenkie de Jong will recover soon, so Barcelona only has 1 injured midfielder. It won't be a very serious problem for Xavi, he can replace him with other midfielders. However, we expect that Gavi won't get injury for a long time.
Buying players may not be priority for Barcelona this season especially as they are not financially boyant enough to shoulder such bill they may rather just have to improve their performance with the team they have got already that way it will save them cost and they will optimize their team depth.
They have still got good players after all except for the case of lewandoski they have almost little to no case of injury this season so most of their players are still in good form and strength to be able to make sure they get the best out of this season. Barcelona just needs to further work on their tactics and they players positioning aswell as rotation when possible amongst players, that way they get to maximize the squad except they may opt in for a loan where necessary.