Apart from the midfield, juventus also has a pretty bad defense so far, just imagine they have conceded 15 goals from 12 matches in the league and 2 goals in the champions league,
Those defensive problems are also connected to the weak midfield. Mistakes made with passing the ball around, lost balls 30 meters from your own goal, and things like that. With a stronger and more compact midfield, few balls would go through for the defenders to deal with.
in my opinion, if juventus still wants to get back on a positive track and be able to compete in the big four Serie A standings this season, they must really be able to take advantage of the summer transfer market to improve their squad and one of the things Juventus must focus on is their defense which looks quite fragile.
You probably meant that they have to take advantage of the winter transfer market. It will already be late to wait until summer, and that's the end of this season.
One possible addition that was mentioned in the past is Rudiger from Chelsea. His contract is expiring at the end of the season and he already featured in Serie A for Roma. He knows his way around. Chiellini has unfortunately gotten old and injury prone. Rugani is useless for a team of this caliber. They are basically relying on the duo Bonucci and de Ligt alone. That simply can't work.