The success saw the Italians top the table on straight on outcomes with Barca, leaving Lionel Messi and Co. to agree to the second put on a disillusioning night at the Camp Nou.
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It was Ronaldo's first gathering with Barca since leaving unpleasant opponents Real Madrid in 2018 and it didn't take long for the forward to transform the game.
Portugal worldwide tried Barca goalkeeper Marc-Andre Ter Stegen almost immediately, prior to opening the scoring from the spot in the thirteenth moment subsequent to being felled by Ronald Araujo.
Protector Danilo streaked an exertion only wide as the guests wrenched up the weight, with a staggering McKennie volley putting the Italian heroes two objectives up and in full control seven minutes after the fact.
"The thrashing was a direct result of how we played in the initial 25 minutes," an irate Barca mentor Ronald Koeman said.
"We began seriously, we looked terrified, we weren't forceful. It looked as though we'd come out looking not to lose, rather than attempting to control the game. We weren't acceptable, not even positionally.
"We improved yet we lost the game in the principal thirty minutes."
Ronaldo got his second punishment when Clement Lenglet was punished for handball seven minutes after the restart.
Things nearly deteriorated for the Catalans a fourth of an hour from the time when Leonardo Bonucci thought he had scored, just for a VAR survey to show he had been offside as he packaged the ball in from short proximity.
The outcome puts the enduring an onslaught Koeman under tension, with the club languishing consecutive annihilations over the first run through under his stewardship.
Forward Antoine Griezmann, be that as it may, said the destruction was not Koeman's issue.
"Who else's flaw might it be able to be other than the players? We're the ones who go out there and play," he said.
"We were missing everything: want, disposition, a readiness to run. Everything was awful.
"We've left a truly downright awful here this evening, for ourselves and for the fans."
