Thierry Henry believes the El Clasico loss will create some doubts at Barcelona ahead of their Champions League quarter-final against Atletico Madrid.
Luis Enrique's men saw their 39-match unbeaten run come to an end with a 2-1 loss to rivals Real Madrid on Saturday.
They remain in control of the La Liga title race with a six-point advantage at the top with seven games remaining in their season.
But Henry, who spent three seasons at Barca, believes there would be some question marks at Camp Nou ahead of Tuesday's first leg at home to Atletico.
"Knowing Barcelona and the Catalans there will be a little doubt," the Frenchman told Sky Sports.
"Just one loss can create doubts, we will see in the coming week. I still think the league is done, but I want to see the impact in the coming games.
"Real deserved the win. In the second half they were the better team, even going down 1-0, then down to 10 men. Real come out with a lot of confidence while Barcelona have to play Atletico next."
Henry was full of praise for Madrid coach Zinedine Zidane, whose team came from behind and had Cristiano Ronaldo score the winner when they were down to 10 men.
"I like the way Zidane set up the team," he said.
"They were not scared, they put pressure on when they needed to and kept their composure at 1-0 down.
"They were trying to play from the back under pressure and they looked like a team with the right balance. Zidane understood he had to take care of the midfield, especially against Barcelona. People wanted to know if tactically he could do something against Barcelona and he did."