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Ante Budimir scored twice, while Bryan Zaragoza and substitute Abel Bretones were also on target for the hosts, who claimed their first league win over the Blaugrana in nine attempts.

Osasuna was unbeaten in its six previous home league games and took an 18th-minute lead when Budimir nodded home from Zaragoza's cross.

The latter, who is on loan from Bayern Munich, doubled the advantage 10 minutes later when he rounded off a swift counter.

Pau Victor halved the deficit eight minutes after the restart, but Budimir's 72nd-minute penalty put the hosts back in control, before Bretones made it 4-1 with seven minutes remaining.

Lamine Yamal climbed off the bench to reduce Barca's arrears, but it was little more than a consolation for Hansi Flick's side, which dropped its first points this term. 

Flick was aiming to become only the second Barca manager after Gerardo Martino in 2013-2014 to oversee eight straight victories at the start of a LaLiga season.

However, he and the visitors were denied by their inspired hosts, who have now recorded four successive home league wins for the first time since September 2011.

Flick took the blame after the match having made a call to rest some of his key players.

"You have to accept these defeats. We didn't play very well. I think it's my responsibility," Flick said.

"[With the changes] I tried to protect the players, because they have been playing a lot of minutes. But I didn't expect us to play like this.

"We made a lot of mistakes and Osasuna did well. Although in the second goal I think there was a foul at the start of the action, I'm not entirely sure because I haven't been able to see it yet, but that's what I've heard.

"To score four goals against us is too much, but I've told the team that we have to keep going. It's a busy schedule and we are on the right track."

Flick was referring to a challenge by Osasuna midfielder Lucas Torro, who stood on Pau Victor's foot, in the build-up to Zaragoza scoring their second goal.

The goal was given as neither the referee nor the VAR saw an infringement.

"For me it is a clear foul," a visibly frustrated Pedri said.

Pedri also shrugged off the suggestion Flick's changes were the cause of Barca's issues.

He added: "It's normal to make rotations, there are a lot of matches. It's no excuse. We had to go out with 11 players and the score at half-time penalised us.

"The first half we were not good. We didn't come out to the game well. In the second half we deserved a bit more, but goals came when we were at our best.

"We had to go for the game. We had to go for everything and on the counter-attack they hurt us."