The glorious return of Pep Guardiola to Barcelona was not quite so glorious. A 4-0 defeat for his Manchester City side was humbling, embarrassing and hurtful. It has left the Citizens with almost no chance of topping their group and with Claudio Bravo suspended for the return fixture.

Also returning to his former club, Bravo committed goalkeeping suicide during the second half with an idiotic handball outside the box after some poor judgement with the ball at his feet. City were not played off the pitch, however, and can take some positives.

It was largely down to Manchester City's own mistakes that they conceded four and Barcelona are one team that you cannot afford to give free passes away - especially towards your own goal. Lionel Messi was utterly phenomenal throughout and notched yet another hat-trick.

Licking of wounds, deep analysis and some blame games will take place between now and their next match in the Champions League, but what did we learn about the Citizens?

Crumbling defence

FC Barcelona v Manchester City - UEFA Champions League Group Stage - Group C

Despite turning in some relatively calm, assured defensive performances earlier this season, the frailties of Manchester City's back line were exposed once again.

Tottenham and Celtic managed to trouble the defence and Barcelona, with all the wonderful players in their side, were just far too good.

Bravo teething troubles

FC Barcelona v Manchester City - UEFA Champions League Group Stage - Group C

Pressure has been on Claudio Bravo since day one and it will be far heavier on his shoulders from now on. A moronic moment cost his side the game, ultimately, and his sending off led to the humiliation of his team-mates.

How we would love to be a fly on the wall next time Guardiola has a chat with the Chilean.

Too bold, too soon

FC Barcelona v Manchester City - UEFA Champions League Group Stage - Group C

With City still yet to fully adapt to Guardiola's style of play, opting to play without their talismanic, prolific goalscorer was always going to be questioned in the event of defeat.

Aguero's absence meant a change in the way they played in possession and City struggled to make such adaptations to their game, particularly in the first half. The Argentine's presence would not have compensated for the defensive humiliations, but it would have allowed for more balls directly in behind the Barca defence.

Where's Vin?

Swansea City v Manchester City - EFL Cup Third Round

Rumours fluttered around that Vincent Kompany could have been available for this fixture, but the Belgian club captain was not even on the bench.

There is no doubt that a fully fit company makes City a far better defensive unit, yet his persistent injury woes bring his future at the club into doubt.

Things would certainly have played out rather differently this evening if Kompany was around.

Full-back pain

FC Barcelona v Manchester City - UEFA Champions League Group Stage - Group C

Manchester City's weaknesses at full-back have been endlessly publicised and they really paid for their struggles in wide defensive areas against Barcelona. Although Zabaleta eventually suffered an injury, both he and Kolarov were suspect when out of possession throughout.

One of the few issues that was not addressed in the summer, the Citizens must wonder how this can be fixed before January.