Coach Ernesto Valverde's selection decisions have been questioned in the aftermath of the defeat, with Lionel Messi left on the bench and major signing Antoine Griezmann seemingly instructed to play on the wing.
The reigning champions lost 2-0 and are languishing in the early table after taking seven points from their opening five matches, winning twice.
Barca have made their worst start to a season for 25 years and are without an away victory in eight since a win at Alaves in April, losing five of those matches.
Here, Sports Mole looks at how Barca could line up for Tuesday's La Liga match.
Versatile left-back Junior Firpo deputised for the injured Jordi Alba against Granada, putting in an unconvincing performance in a defence on the back foot from kickoff.
Culpable for the lightning opening goal, Firpo was replaced by Ansu Fati at half time, and his place could be under immediate threat now.
Winger Carles Perez was also withdrawn at half time, Valverde yielding to temptation by bringing on Messi in an attempt to rescue the game.
Messi and Fati could start against Villarreal, although the loudest question probably concerns where Griezmann will play, having looked marooned and injected little impact on the match from wide.
Ousmane Dembele is not an option, although the forward is not far from returning after a hamstring problem, and classy centre-back Samuel Umtiti will not be available to help improve Barcelona's defence until the end of October because of a foot injury.