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derby disaster the talk among fans was of summer incomings to reboot a blue machine that , all-of-a-sudden , looks as outdated as a Commodore 64 .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 .
Manchester City were beaten 4-2 by Manchester United at Old Trafford . Defeat leaves City 100th 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 .