Sergio Aguero: Man City striker proves he is priceless against Monaco

Sergio Aguero took his tally to 20 goals for the season

Manchester City's heady mix of fantasy football and flawed defending was on display for all to see in the thrilling Champions League victory over Monaco at Etihad Stadium.

Pep Guardiola's side were irresistible going forward and an open door at the back as they secured a 5-3 first-leg win in the last 16, the highest-scoring game at this stage of the tournament's 25-year history.

City's performance still leaves them with questions to answer when they confront this gifted Monaco side in the second leg on 15 March - but one fact remains without dispute in this Guardiola era...

City cannot live without Aguero

Leroy Sane was the stand-out performer in one of the finest matches seen at Etihad Stadium - but the cutting edge was provided by 28-year-old Argentine Sergio Aguero, whose future has come under scrutiny in recent weeks.

The spectacular and instant impact of Brazilian teenager Gabriel Jesus saw Aguero left on the bench for the Premier League victories against West Ham, Swansea City and Bournemouth.

It is on nights like this, however, where Aguero demonstrated that the idea City would somehow be better off without him is a nonsense.

Jesus, at just 19, and 21-year-old Sane, represent a golden future for Manchester City along with Raheem Sterling at 22, but Aguero looks like a player in his prime and fit to play his part - not just now but in the years ahead.

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Aguero has no point to prove, his record speaks eloquently enough, but he has accumulated a reputation and body of work that makes the finest defenders fear his threat. And at the elite level of the Champions League, that is a priceless commodity.

It was a match that was arguably the most enthralling seen here since Aguero's 94th-minute winner in May 2012 which secured City their first title in 44 years. And the striker showed he still retains all the old powers.

Aguero and his City colleagues had to contend with a blaze of attacking intent from Monaco, but in between the youthful zest of Sane and Sterling, he was the spearhead and the creator.

He was rewarded for trying his luck with his first goal that prompted a dreadful error from Monaco keeper Danijel Subasic. He scored his second with a crisp, instinctive right-foot volley and then set up the fifth for Sane, the goal that gave City a cushion. It was the complete, consummate attacking performance.

Aguero also has a psychological impact on opponents and it gives Guardiola and City a powerful weapon. His first goal was City's 200th in the Champions League and he has now scored five in his past three games at Etihad Stadium in this competition.

Yes, his recent goalscoring form has not been of his usual standard but he is the epitome of the phrase "class is permanent".

The Catalan's decision to shunt the popular goalkeeper Joe Hart out to Torino on loan was taken on the chin by supporters who idolised him. Any similar fate for Aguero would not meet such easy acceptance.

The manager's embrace and kiss on Aguero's forehead when he left the action late on Tuesday evening was surely a gesture of appreciation that should end the speculation - although this latest masterclass must have done that anyway.

And the bottom line is this.

How much would it cost City to even think about replacing Aguero? A lot more than the £38m it cost them to bring him in from Atletico Madrid in July 2011.

Man City flaws leave tie in balance

Amid the fanfare and celebration at the conclusion of a quite magnificent game of football, Manchester City were accompanied on their way to a warm weather break in Abu Dhabi by notes of caution.

Yes, they have a crucial two-goal cushion, which Guardiola would have taken gratefully before kick-off - but an examination of City's defending and Monaco's wonderful attacking pace, movement and threat tells you this may yet be a flimsy advantage.

Guardiola knows the score. Park the bus with this defence and Monaco will have that particular vehicle off the road and rolling away into the distance with ease.

This will not be City's way, as Guardiola said: "We will fly to Monaco to score as many goals as possible. If we don't score in Monaco we will be eliminated."

Read that as code for saying 'City have got no chance of keeping a clean sheet'. This reality has been grasped by Guardiola - helped by the fact that Monaco are the highest scorers in the top five European leagues with 111 goals in all competitions.

He said: "If one team can score a thousand million goals it's Monaco. They arrive with six or seven players in the box and it's tough to control that on the counter attack."

And if there is a team that is vulnerable to such a threat it is City.

Guardiola's decision to play Fernandinho at left-back and leave Yaya Toure isolated was flawed - can he really repeat such a tactic in Monaco?

Guardiola was, at times, frantic in his technical area. One cry of anguish as possession was conceded in a dangerous area was so pained it was plainly audible at the back of the lower tier of the stand behind him.

Goalkeeper Willy Caballero's poor distribution led to Radamel Falcao's headed equaliser, while brilliant teenager Kylian Mbappe's rasping finish for Monaco's second saw City's defence split by a routine long ball.

Falcao's second and Monaco's third was brilliantly executed but easily created as John Stones showed an alarming lack of strength and determination when challenging the Colombian.

Guardiola must find a way to eradicate these flaws and somehow make City a tougher proposition by 15 March or this dangerous, predatory, pacy Monaco side will make short work of a two-goal deficit.

And Manchester City's manager knows it.

Guardiola still has keeper conundrum

Caballero, it can now be assumed, is Manchester City's first-choice goalkeeper. Claudio Bravo, Guardiola's chosen one when he was brought in for £17m from Barcelona last August, has been demoted.

Caballero was given the starting place on Tuesday after Bravo, who has earned a reputation as the shot-stopper who does not stop shots, was on FA Cup fifth round duty at Huddersfield.

The 35-year-old Argentine is surely only a sticking plaster solution for a bigger problem facing Guardiola, one he must address in the summer.

This means there is an element of muddling through some potentially vital games between now and May and Caballero delivered a decidedly mixed bag against Monaco.

Willy Caballero has now established himself as Manchester City's number one

The ability, or lack of, to play with the ball at his feet was among the factors that apparently did it for Hart and prompted the signing of Bravo.

It is a central plank of Guardiola's philosophy and one that cost City the first goal against Monaco. The manager's leaping, head-in-hands reaction was a sign he knew trouble - in the shape of Falcao's header - was coming from the moment Caballero sent out a poor clearance.

Caballero was only obeying orders but it was clear on several occasions he is not comfortable with them.

He redeemed himself with a save from Falcao's penalty; the striker hesitating so long over the spot-kick he almost seemed to forget what he was supposed to do when he arrived at the ball.

Caballero also pulled off a vital stop with his feet from Falcao's late effort with City 5-3 up, an intervention which may prove invaluable in the context of the tie. He made a significant contribution.

There is no doubt, though, that the story of Manchester City's goalkeepers will provide a narrative between now and the end of the season.