Pep Guardiola have been asked on few occasions of he'll be making some signings in the January transfer window so he can balance his team but his response have always been that he "wants his injured players back". Just like you said, the Spanish manager needs to make some significant additions to the team's so they can be competitive again but if he decides not to do that, I'll not be surprised to see Manchester City not finish the season in the top four.
Pep Guardiola can not buy players without given budget from the club owner and there is barrier and risk that if they overspend, they will be punished by UEFA soon. There is rumor that Manchester City are facing with big risk of a severe punishment from UEFA with financial issues in their club latest years. Fraud financial reports in the past, can cause massive harms on Manchester City in future.
With this pressure, their club owner will not want to give Pep Guardiola too much money for winter transfer market. The coach will mostly base on available players include young ones, to lead his club going through this hard time as best as possible.