Juventus are weak defensively which have been a major cause in their drastic drop in form and resulting in poor games in the league and in the Champions league. Their performance in the league have been much better than in the champions league which seems to me that to evaluate a teams performance, you will get the right answers when they compete in European top level like in the Champions league or Europa league because you are competing on a different level there against the best in Europe. That could probably be the reason why they play better in the domestic league and not in the Champions league. However, they need to strengthen their defense to stand a chance in the Europa league.
Juventus have had very different performances in Serie A and the Champions League. In Serie A, Juventus' defense is very good with only conceding 7 goals in 15 matches, but on the other hand their defense is very weak in the Champions League where in 6 matches they conceded 13 goals.
Basically I don't know Juventus' biggest obstacle this season in Europe other than the injury of several important players, but they have to prove something in a better way in the Europa League later. I don't think Nantes will be a strong opponent for Juventus in the playoffs, so Juventus will have to win that match to qualify for the round of 16.