I'll leave others to use better knowledge of the european scene and suggest players, but I look at it this way. Our match day squad when we beat Brighton at the end of last season contained 4 centre half's, Kompany, Otamendi, Laporte and Stones.
One of them left, City didn't replace or promote a youngster, City decided to go with the three remaining. One of which, Otamendi, was reported to be leaving as well, another, Stones, had been injury prone for the previous two seasons. Then our best defender, Laporte got long term injured. We messed up in the summer.
Spend on two centre half's in January, as we did with Laporte, use the rest of this season to get them embedded in the squad and Pep's ways, as at this rate the league could be gone by then. Otamendi should be sold next summer.
Next summer we should be deciding if our trio of left backs are actually trusted or fit enough. We'll need an additional midfielder with Silva going, Yes Foden's coming through, but Fernandino is a year older. We may need a winger is Sane goes, and Sergio isn't getting any younger either.
The re-building never stops when your on top. City have work to do.