Nico Williams' return to fitness will not solve all of Athletic Club's issues, Ernesto Valverde has warned.

Spain winger Williams returned to Athletic's line-up for the first time in a league match since February, but could not help Valverde's team avoid a 2-1 defeat to Villarreal.

Williams has been nursing a groin injury, and after making a substitute appearance against Getafe, got the nod to start on Sunday, featuring for 66 minutes. He created a team-leading two chances, while successfully completing five of eight dribbles, both of which were game-highs.

The 23-year-old has scored four goals and provided two assists in LaLiga this season, but Valverde does not want to be too reliant on one player.

"Nico has been sidelined for a long time and playing through discomfort," Valverde said.

"Now we can't think that just because he's back, everything is solved. We also had a very tough opponent."

Boos from the home fans could be heard late on, as Athletic pushed for an equaliser. They sit in the bottom half of LaLiga and a European qualification push looks like a tall order.

"The atmosphere on the pitch wasn't very good," Valverde conceded.

"If you don't win, the fans don't like it; you have to win.

"It was a very tough match for us. They scored the first goal in an incredible play. Even so, we had chances to equalise with our pressure, and it was a shame about the last play of the first half with the score at 0-2.

"We could have scored, we pushed forward, taking a lot of risks, with some nervousness, but it wasn't to be."

Athletic have lost four of their last five league matches, while Villarreal moved four points clear in third place with their victory.