#1 Lack of creativity from the midfield
Barcelona have usually used a three-man midfield since the Pep Guardiola era. The Blaugranas then had Xavi, Iniesta and Busquets, a trio which made the finest of teams struggle. Xavi's control, Busquets' interception and Iniesta's creativity ruled the midfield every time Barca were on the field. But ever since the departure of Xavi the dominance in the midfield seems to have faded. Iniesta found it difficult to link up with the other midfielders after Xavi's departure.
Ivan Rakitic was chosen as Xavi's heir to the throne. He showed his excellence in his first season at the Camp Nou but has failed ever since. The Croatian has turned into a very conservative player who hardly takes any risk and fails to provide lethal balls in the final third which is primarily due to a vulnerable right back position.
Busquets' main job has been to connect the midfield and defence and he has been doing it pretty well even after Xaviesta's departure. Rakitic not being press resistant has resulted in Messi dropping deep to create chances.
These problems were generally not seen in the La Liga because the small teams did not tend to press high but the teams in the Champions League have exploited Barca's midfield problems to snatch victory away from them.
Arthur Melo's arrival has however somewhat solved the problem this season and Barca will be hoping for a more stable midfield for their game at the Old Trafford.