The main problem Manchester City faced in their past two months was a defensive problem which exposed the team and make them dropped from the first position in the Premier League table to where they are now; they have recovered a bit in their last two matches.
Manchester City's main problem is not the defence, it's the midfield. They don't have a hardworking midfield. A midfield should consist of a ball-winner and technical players. Rodri was the ball winner and he was also very technical on the ball so he was very crucial for them. Since he got injured, the midfield has been exposed.
Getting a defender will help Manchester City a lot, but they still need a midfielder. If they cant get a natural DM, they can try to get a central midfielder that is hardworking and a ball winner, that way they can have control of the midfield. They also need a winger too.