Hat-trick marks miserable Nou Camp return for Guardiola
Barcelona 4-0 Manchester City
Not even Pep Guardiola can wake Manchester City from their recurring Barcelona nightmare - the one where Lionel Messi plays the uncontrollable bogeyman.
For the third time in three visits to the Nou Camp, City emerged beaten, reduced to ten men and picked off by Messi, who treated his old boss to a hat-trick on his first return to Catalonia with his new club.
When Guardiola, Barca's glorious coach between 2008 and 2012, returned for the first time with Bayern Munich 18 months ago, he was on the wrong end of a 3-0 defeat, but it was worse this time.
Barcelona and Messi were good, make no mistake, but City were masters of their own downfall with each of Messi's goals coming as a result of the visitors' failings.
A Fernandinho slip led to the first, Messi was left unmarked for his second and only John Stones knows why he froze like a statue when Luis Suarez skipped around him to tee up Messi for his third.
With goalkeeper Claudio Bravo sent off for deliberate hand-ball - Barca defender Jeremy Mathieu was later red-carded for a wild tackle on Raheem Sterling - City ended up with ten men for the fourth time in five games against Barcelona.
And on a night when Guardiola chose to start without Sergio Aguero, everything that could go wrong did go wrong for City until Willy Caballero saved faced with an 86th-minute penalty save from Neymar. Even then, Neymar atoned with Barca's fourth a couple of minutes later.
Guardiola has not been shy in making big decisions since beginning work at City this summer, as Joe Hart - shipped out on loan to Torino - will testify. Omitting Yaya Toure from the club's Champions League squad was another surprise move, and the coach has also shown little desire to rush captain Vincent Kompany back into the team, despite the Belgian returning to full fitness.
Hart, Toure and Kompany had become a strong unit at the club, with only David Silva and Aguero coming close to matching their importance and standing in the dressing-room, but there is now only one boss at the Etihad and that is Guardiola.
No manager would choose to drop Aguero without being as powerful as Guardiola has already become at City, but that was the decision taken and he made no effort to palm it off as the Argentinian being rested.
Guardiola insisted pre-match that Aguero's omission was tactical - "I want more midfielders" - and City's all-time leading Champions League scorer was benched in favour of a three-pronged front-line of Kevin De Bruyne, Nolito and Raheem Sterling, all of whom were expected to flood midfield when Barcelona had the ball.
Was it bold? Was it risky? Probably both, but the tactic ensured that City were able to trade blows with Barcelona in a feisty first half. Messi's goal was an example of his brilliance, and it separated the two teams, but City created their own chances and they certainly held their own in between phases of Barca dominance.
But when Luis Enrique's team were on top, City were stretched, with the likes of Fernandinho, John Stones and Sterling all losing the ball carelessly in dangerous positions.
Messi's goal, his 13th in his last 13 games against English opposition, came following another example of a lack of focus on possession, when Aleksandar Kolarov was dispossessed by Javier Mascherano.
Mascherano fed the ball to Messi, who scampered towards the area before attempting to pick out Andres Iniesta in the six-yard box. Pablo Zabaleta beat Iniesta to the ball, which looked set to be cleared easily by Fernandinho until the Brazilian slipped.
Slipping was bad enough, but one can only imagine the horror Fernandinho felt when he raised his head to see Messi racing down on the loose ball.
It was an easy kill for the Barca forward, who dummied Bravo before sliding the ball into the net for his 87th Champions League goal.
City responded well, however, with Guardiola's team deciding that attack was the best form of defence.
Nolito and Ilkay Gundogan both forced important saves from Marc-Andre Ter Stegen before Stones missed a golden chance on the stroke of half-time when he headed wide from six yards.
It could have been worse for Stones just 30 seconds later when Suarez attempted to lob Bravo, only for the goalkeeper to tip the effort over the bar.
City then claimed a penalty, rejected by Serbian referee Milorad Mazic, when substitute Lucas Digne appeared to handle a Sterling cross.
As much as City were still in the game at the start of the second half, the magnitude of their task in terms of fighting back to win was summed up by Barcelona's remarkable record of not having lost a Champions League at the Nou Camp while leading at half-time since Bayern Munich overturned a first-half deficit in November 1998.
So City already had a mountain to climb before Bravo turned it into Everest by giving referee Mazic no option but to dismiss him on 53 minutes.
Three minutes earlier, Samuel Umtiti had gone within inches of doubling Barca's lead with a header which flew narrowly wide, but Suarez believed he would do what the French defender failed to do when Bravo's mistake gifted him the ball 50 yards from goal.
Bravo takes too many risks and Barca have clearly come out on top of the deal which saw Ter Stegen stay and the error-prone Chilean move to City.
On this occasion, losing possession to Suarez left Bravo 40 yards from his goal and, when the forward took aim, the keeper handled the ball. It was a clear red card and he was off.
Replacement keeper Willy Caballero breathed a sigh of relief when Messi sent the subsequent free-kick over the bar, but the little forward was only delaying the pain for Bravo's understudy, as seven minutes later, he made it 2-0 with a low shot from 15 yards after being released by Iniesta.
Having lost Zabaleta to injury, City were now playing with ten men without one of their most experienced defenders and half an hour to see out.
City continued to search for a goal, despite their one-man disadvantage, and De Bruyne and Stones both went close to scoring.
Instead, two more goals came the other end. Messi stroked the ball in from six yards on 69 minutes after Stones had been caught out of position before Suarez raced around the back to tee up Messi.
Messi would have had four had he taken the penalty he won after being fouled by Kolarov, but he handed the ball to Neymar, only for the Brazilian to see his spot-kick saved.
Barcelona never rest, though, and Neymar was determined to make amends, which he did when scoring a fourth after slaloming through the City defence in the 89th minute.
So four games and no wins now for City and Guardiola. It is hardly a crisis, but it is certainly a challenge the most celebrated coach in world football must face to get his team back on track.