Manchester United have already made two big-name signings this summer. The Red Devils will need to wait a bit before adding a third as their attempt to lure another world-class center-back to Old Trafford has proved futile.According to a report, Manchester United made a bid of €30 million to sign Napoli defender Kalidou Koulibaly but their approach was immediately turned down by the Serie A club who have made it clear that the player isn't for sale.Kalidou Koulibaly | Journalist says “Manchester United have offered €30m” – Player keen on joining ‘prestigious’ Red Devils. Offer is this summer, not clear if it was before Varane deal.https://t.co/xlu3YeVq1Y #mufc— Sport Witness (@Sport_Witness) August 4, 2021Sources claim the Senegalese center-back was keen to join the Red Devils but Napoli CEO Aurelio De Laurentiis blocked the transfer as he still considers the defender a vital member of the squad.Koulibaly has been one of the top center-backs in Serie A for the past few years. The 30-year old made 26 appearances for the Parthenopeans in the Italian top flight last season, recording 36 tackles, 29 interceptions, 57 clearances and 26 blocks to his name. (Stats: Whoscored).Bayern Münich are not working to sign Kalidou Koulibaly, as of today. Premier League clubs are still interested as last summer, Bayern have not started any talk with Napoli or Koulibaly agents atm. 🔴 #Bayern #EPL— Fabrizio Romano (@FabrizioRomano) March 8, 2021The defender has been the subject of intense speculation over the last few years but has always ended up staying put in Naples. With Manchester United's approach reportedly snubbed instantly, everything suggests he'll continue at the Stadio San Paolo next season.The defender was interested in the moveDo Manchester United really need Koulibaly?While Koulibaly is, without a doubt, one of the finest center-backs in Europe at the moment, Manchester United's interest in the player's services is difficult to understand. There is a big question as to whether the presence of the Senegalese is needed at Old Trafford.The Red Devils recently reached an agreement to sign Raphael Varane from Real Madrid and the Frenchman is braced to form a formidable partnership with Harry Maguire at the heart of the defense next season. Eric Bailly and Victor Linedolof will be on standby to provide competition.It would've made more sense to divert that approach in favor of a midfielder. Paul Pogba is reportedly nearing an exit from Old Trafford. A decent replacement needs to be signed to strengthen the options in the middle of the park. It remains to be seen if the Red Devils will turn their attention to where it really matters in the coming weeks.