Barcelona's Lionel Messi celebrates scoring his third goal in Wednesday's 40 Champions League Group C victory over Manchester City at Camp Nou stadium in Barcelona. It was Messi's second straight Champions League hattrick. [Photo/Agencies]

Manchester City coach Pep Guardiola was defiant he will not change his coaching philosophy after watching Barcelona score three times against his back-up goalkeeper en route to an easy 4-0 victory on Wednesday.

With City trailing 1-0, starter Claudio Bravo was sent off for thwarting Luis Suarez outside the area after gifting the ball to the Uruguayan striker.

That put the spotlight on Guardiola's decision to jettison veteran England keeper Joe Hart.

Bravo was replaced by Willy Caballero, who looked shaky at times but saved a penalty from Neymar.

"Until the last day I am a coach I will try to build my teams from the goalkeeper," said Guardiola.

Bravo enjoyed two almost faultless years at Barca before joining City in August. He has endured a shaky start under Guardiola's plan to build City's attack from the back.

"He (Bravo) has a lot of experience, he is one of the best goalkeepers in the world for the last 10 years, but he will learn and he is the first one in the room to apologize," said Guardiola.

Bravo's gaffe was just one of many City defensive howlers on the night as Messi took advantage of Fernandinho's slip to give Barca an early lead.

Messi curled home a second before Ilkay Gundogan's misplaced pass went to Suarez, who squared for Messi to complete his hat-trick.

The scoreline could have been even more embarrassing for City had Caballero not saved a penalty from Neymar before the Brazilian made amends with a brilliant fourth.

"It is always difficult to play in Barcelona with 11 players, and with 10 the game was over," said Guardiola.

"We created enough chances, we arrived at the byline many times, but with their strikers they punish you."

Guardiola won his first 10 games at City's helm, but the club has now failed to win any of its last four matches and the Catalan lamented the series of error-strewn performances affecting his side.

A 3-3 draw at Celtic was followed by a 2-0 loss at Tottenham Hotspur before City missed two penalties in a 1-1 draw with Everton last Saturday.

"Starting in Glasgow, own goals in White Hart Lane (against Tottenham), missing penalties ... we've given a lot to our opponents," the manager said.

Guardiola, who won 14 trophies in four years in charge of Barca, has now lost both times he has returned to Camp Nou after losing 3-0 in 2015 when he was with Bayern Munich.

However, Guardiola's former teammate and Barca boss Luis Enrique agreed Bravo's dismissal changed the game.

"The sending-off was decisive and conditioned the game a lot," said Enrique.

"Teams that try to play from the back know those errors can happen and you have to accept it. Mistakes happen, but you must be prepared for that."

Messi has now scored two hat-tricks in as many games in this season's Champions League as the Argentine showed no signs of rust on his first start for a month after a groin injury.

"We have seen the calm that Messi has to finish inside the area as if he was in the school yard," added Enrique.

"If there is anything you can expect of Messi, it is that."