Manchester United fans would have been filled with hope and positivity after a superb end to the 2019-20 Premier League campaign. Manchester United started the new year in eighth place, but a run of 14 unbeaten games in the Premier League, saw the Red Devils finish in third place.The tables have turned at Manchester United after their dismal start to the 2020-21 Premier League campaign, which has seen them lose two of their opening three fixtures. Manchester United's woes were compounded by their shocking 6-1 thrashing at the hands of Tottenham Hotspur.Most of the criticism that Manchester United received after the Spurs game was aimed at their defensive display, in particular the performance of their club captain Harry Maguire.Harry Maguire was signed by Manchester United for £80 million from Leicester City in the summer of 2019, a fee that made him the most expensive defender in football history. Manchester United hoped that Harry Maguire would have the same impact at Old Trafford as Virgil Van Dijk has had on Liverpool.Manchester United were expected to sign a top-quality centre-back this summer to pair with Harry Maguire in the heart of United's defence, but failed to do so. The 20 time champions of England are now in a situation where they may need to sign someone to replace a defender they spent £80 million to sign just last summer.Harry Maguire's apology to the fans pic.twitter.com/s2otbxMOSG— utdreport (@utdreport) October 4, 20205 players who could replace Harry Maguire at Manchester United#5 Dayot UpamecanoDayot Upamecano Dayot Upamecano is one of the biggest young prospects in Europe. The 21-year-old centre-back was heavily linked with a move to Manchester United this summer. He was also linked with moves to Barcelona and Arsenal.The French centre-half is a physical specimen, whose game is based on raw strength and speed, attributes which would stand him in good stead if he were to move to the Premier League.Upamecano will, however, cost United a fortune, as he has just recently signed a new deal with RB Leipzig, which would keep him at the club till 2023. The France international would be a worthy investment for Manchester United given his age, and his experience in European competitions for Leipzig.📝 — Manchester United retain an active interest in the 21 year old French defender Dayot Upamecano, but are highly unlikely to attempt to sign him before the window closes #mufc #mujournal[@sampilger]— United Journal (@theutdjournal) September 28, 2020#4 Milan SkriniarMilan SkriniarInter Milan centre-back Milan Skriniar was the subject of interest from Tottenham Hotspur this summer, who reportedly submitted a €50 million bid to Inter for the Slovakian. Inter Milan were, however, reluctant to let go of their star defender as they seek to challenge Juventus for the Serie A title this season.Skriniar is a defender from the same ilk as Japp Stam, Steve Bruce or Nemanja Vidic. The 25-year-old is a warrior, a man to put his body on the line for his club, which is the type of player Manchester United have craved since the departure of Vidic in 2014.Inter won’t sell Milan Skriniar to Tottenham on last hours of #DeadlineDay. No agreement, no fresh bid after last one [€35m] turned down. Also Spurs sources consider the deal ‘off’. ⚪️ #THFC #Spurs— Fabrizio Romano (@FabrizioRomano) October 5, 2020Skriniar would cost United north of €50 million, but given his style of play, age, and profile, Red Devils could consider pursuing a deal for the defender.