Here are the European teams that disappointed after the conclusion of the 2019-2020 Season.
Disappointments
FC Barcelona

Barcelona ended a terrible season without any trophy. They lost the league title in the last ten matches and were humiliated by Bayern Munich in the UEFA Champions League quarter-finals. The team struggled defensively since their defensive line was fragile and poor. Marc-Ándre ter Stegen was the best defender, saving Barcelona in many occasions. Offensively, they also had issues, expecting Lionel Messi to solve their lack of creativity and inaccuracy in front of goal.
Management have already made some changes, primarily substituting Quique Setién with Ronald Koeman. Setién never knew how to handle the squad nor motivate them. Koeman, on the other hand, wants to make an impact, developing a football plan that includes the dismissal of various players, e.g. Luis Suárez. This has caused discomfort in Messi, wanting to leave the club in this Transfer Market.
Juventus

Even though Juventus won the Serie A TIM, the team suffered a lot during this past season. They narrowly won the league title, lost the Coppa Italia final against Napoli in penalties, and were eliminated by Olympique Lyon in the Champions League Round of 16. Maurizio Sarri was never able to boost his players performance on the pitch; the defensive line was very inconsistent while the midfield poor, incapable of helping the offense or defense.
Management sacked Sarri on his first year as Juventus coach, appointing Italian Legend Andrea Pirlo as his successor. Pirlo, as well as Koeman, has been changing his current squad, deciding to let some of his players, e.g. Gonzalo Higuaín. I believe Aaron Ramsey, Danilo, Daniele Rugani and Federico Bernardeschi are also close to leaving the team.
Manchester City

City could not retain the Premier League title and surprisingly lost to Olympique Lyon in the Champions League quarter-finals. They only won the English Football League Cup (EFL Cup/Carabao Cup), a not very prestigious trophy in the world of football. Their fragile defense and lack of forcefulness in front of goal were their major weaknesses.
Manchester City have bought Ferrán Torres (right winger) and Nathan Ake (centre-back) during this Transfer Market.
Valencia CF
2018-2019 was a successful season for Valencia, winning the Copa del Rey against Barcelona and qualifying to the Champions League. However, this past year has been disappointing.
Everything collapsed at the start of this season, firing coach Marcelino García (management conflicts) and replacing him with Albert Celades. Celades did not even finished the year with Valencia, having a bad record and most of the squad against him. Salvador González was his successor, managing the last 6 LaLiga matches. In the end, the team did not win any title nor qualify to European competitions.
In July, Javi García was appointed as the new coach (third coach in less than a year). Two weeks later, Dani Parejo, the star of the season, decided not to continue with Valencia, making García’s job a bit tougher.
Schalke 04

In 2018, Schalke finished second in the Bundesliga. Now, they were closer to relegation than they were to the title. The German team ended with a negative record of 10 defeats and 6 ties, settling for the twelfth position.
Saint Etienne

Even though Saint Etienne was not able to end the Ligue 1 season due to Covid-19, the team, with the most league titles, had a poor performance, finishing seventeenth in the table with 12 matches to be played.
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