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.
Apart from all the defensive problems the team has faced this season, keep in mind that the team still doesn't have a good forward in the attacking line. Experiments with Chiesa in attack, and the uncertain play of Morata and Dybala is not what Juventus needs. I assumed that when Ronaldo left the team, the club received very good finances, which would be enough for him to strengthen the team. But perhaps Alegre simply decided not to make hasty decisions and is now working on these issues. In any case, Juventus definitely needs a reinforcement by new players in the winter transfer market.