#3 Athletic exploit Barca's lack of width
Barcelona relies on their full-backs to provide the width, especially when both Philippe Coutinho and Lionel Messi play together. Neither of them hugs the touchline, which increases the responsibilities on their full-backs.
That responsibility was not carried out effectively for Barcelona against Bilbao. Neither one of Roberto or Semedo stretched the field effectively, which meant that the likes of Suarez and Messi had to find openings in a congested central area. The lack of width from Barcelona was favorable for their opponents, as they defended in a compact and organized manner.
Too often, Semedo and Roberto were needed in advanced positions, but they were not available. Semedo, in particular, was restricted, playing as a left back. Jordi Alba was missed sorely, and Semedo will surely move over to the right side which provides balance on the wings for Barca.