Although Juventus still have Bonucci and Matthijs de Light as defenders, Juventus must immediately find a replacement for Chiellini. In addition
to considering if one of De Light or Bonucci is injured, Juventus is also less solid if they have to play with a 3 defender formation. Therefore,
one of the main focuses of Juventus should be to bring in a quality defender, so next season the Juventus defense will be more solid. The Juventus
midfield is quite good with the arrival of Paul Pogba and Angel Di Maria, maybe the front line also Juventus should bring in at least one player,
because Juventus lost Dybala and Morata. According to rumors, Juventus are competing for Domenico Berardi from Sassuolo. If it is true that
Juventus managed to get Berardi, this could be good news, because Berardi's performances this season with Sassuolo have been very good.
Of course there has to be a commensurate replacement for a player leaving the team especially if it is a defender. So far there have been some rumors about Juventus' interest in defender Sassoulo, but anyway I haven't heard any recent news about Juventus' interest in the player. Dybala is leaving the team, while Pogba and Di Maria are likely to join. It would be better if Juventus had defenders of good quality, while Bonucci and De Light are two strong strongholds there but they need to get at least one commensurate replacement in case of injury or absence.
I have to agree with your opinion, those are things Allegri needs to consider even though Juventus have good LB and RB. Technically, I think 3 defenders may not be strong enough to defend against attacks from other teams, but usually Juventus' tactic is to have at least 4 defenders, one of whom is placed in the left or right winger position and that player can help the defense.