article: In the aftermath of Sunday's derby disaster the talk among fans was of summer incomings to reboot a blue machine that, all-of-a-sudden, looks as outdated as a Commodore 64. Then there is the inevitable speculation over the future of under-fire manager Manuel Pellegrini. But the biggest challenge for Manchester City this summer could be keeping hold of something they already have. Manchester City striker Sergio Aguero looks on dejected during the defeat by United at Old Trafford . The afternoon started so well for Aguero and City when he opened the scoring after just eight minutes . Aguero celebrates his 99th goal for Manchester City with David Silva as De Gea looks dejected . It may have gone largely unnoticed in the carnage at Old Trafford but there was an important landmark for the brilliant Sergio Aguero that deserves to be recognised. The aftermath of a crushing derby defeat at the old enemy may not be the place for jelly and ice-cream but the Argentine's tremendous feat of scoring 100 goals in just 158 appearances should be cause for celebration. Take a minute to think where City would have been without that century. Remember THAT title-winning goal against QPR in 2012? Most, in that situation, would have put their foot through the ball as soon as it arrived from Mario Balotelli. Not Kun. The man who takes his nickname from a cartoon character had City fans believing in fairy tales when he feigned a shot to open up space before firing home probably the biggest goal in the club's long history. That calm head, that powerful shot and that unnerving ability to find the target are what City paid what now looks like a bargain £38m for in 2011. Aguero netted his 100th goal for City in the 89th minute but there was no cause for celebration . The Argentine strikes in injury time against QPR to win Manchester City the title in 2012 . Aguero celebrates with Edin Dzeko after that goal when City scored twice in injury time to win title . Aguero is settled in Manchester and signed a new five-year deal only last August. However, he now has to think of himself. If the unthinkable happens and City slip outside the top four will one of the best players in the world be happy at a season without showcasing his talents on the Champions League stage? There are admirers. Real Madrid and Barcelona are both fans. Should either register an interest City would face a desperate battle to keep hold of their star man. While a move to Madrid may come with an enticing caveat in the shape of Gareth Bale, the loss of Aguero would be exceptionally difficult to recover from. Where else do you get those goals from? While it is too early in his City career to pass judgement it would appear that the burden would be too much for Wilfried Bony's broad shoulders. Wilfried Bony has only scored once in five appearances since joining City from Swansea in January . City's ageing squad is in need of a major overhaul in the summer . Juventus midfielder Paul Pogba is a potential target as the City owner prepares to open the cheque book . The former Swansea City man, who cost just £10m less than Aguero, has failed to hit the ground running since his January move and has scored only once in five appearances. The powers-that-be would no doubt have to open their chequebook but who would come to an Aguero-less City? And, should they sleepwalk out of the top four, who would dream of coming to a club without Champions League football? The likes of Paul Pogba and Raheem Sterling are potential targets. Sportsmail understands discussions have already taken place with a number of new additions. But will they also want to come to a club shorn of its idol? It is a big question and one that City will be desperate not to have to discover the answer to. Liverpool contract rebel Raheem Sterling could also be on any list of summer targets . Manuel Pellegrini is facing the sack even if Manchester City qualify for the Champions League . There are others to ponder. Does Pellegrini stay? And what share of the blame do executives Ferran Soriano and Txiki Begiristain take? Are they at fault for spending the best part of £200m since 2012 without really strengthening the team or does Pellegrini carry the can? With Financial Fair Play sanctions hurting, it was essential that City got bang for the buck. In reality they barely had a whimper. The pile on the in-tray on chairman Khaldoon al Mubarak's desk seems to be getting taller every week. It is essential that keeping Aguero is top of that ever-growing list or else that ageing blue machine risks heading for an even bigger crash.
highlights: Manchester City were beaten 4-2 by Manchester United at Old Trafford .
  Defeat leaves City fourth in Premier League, four points behind Man United .
  Man City desperately need to strengthen an ageing squad in the summer .
  And it remains to be see whether City will stick with Manuel Pellegrini .
  But the biggest challenge could be keeping hold of Sergio Aguero .
  Argentine scored a brace against United to take his tally for City to 100 .
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