Manchester United are the most successful team in the Premier League era. With 13 triumphs, they have as many Premier League titles as Arsenal (3), Chelsea (5),and Manchester City (5) combined.All their title triumphs in the Premier League era came under Sir Alex Ferguson. The Scottish legend established United as a force to be reckoned with during his stay at the club. However, since his retirement in 2013, the red half of Manchester has struggled.Manchester United's transfer-market woes are well documentedSince the departure of Alex Ferguson, United have fallen off the perch domestically. It has been almost a decade since the 13-time Premier League winner left Old Trafford, but the Manchester-based club are yet to win the league.Much of their failures are down to a poor recruitment policy. Over the last few years, the Red Devils have made too many poor signings.Football Tweet ⚽@Football__TweetManchester United's number 7 shirt since Cristiano Ronaldo left in 2009:Michael Owen: 5 goalsAntonio Valencia: 1 goalÁngel Di María: 3 goalsMemphis Depay: 2 goalsAlexis Sánchez: 3 goals3:33 AM · Aug 31, 201934776Manchester United's number 7 shirt since Cristiano Ronaldo left in 2009:Michael Owen: 5 goalsAntonio Valencia: 1 goalÁngel Di María: 3 goalsMemphis Depay: 2 goalsAlexis Sánchez: 3 goals https://t.co/YFmN12VqX6On that note, here's a look at five of their biggest flops in the last 30 years.#5 Diego Forlan - €11 millionManchester United vs MiddlesbroughDiego Forlan is one of the rare cases of a great striker failing under Sir Alex Ferguson. The Uruguayan was bought from Independiente for €11 million, which was a substantial amount in 2002. Forlan found the net in 13 Premier League games and five Champions League appearances during his first season at Old Trafford.However, his goal record over the next few seasons left a lot to be desired. Forlan scored just ten goals in 49 league outings in 2002-03 and 2003-04. These numbers were unacceptable for a centre-forward playing for a title-contending side.Squawka Football@SquawkaON THIS DAY: In 2004, Diego Forlan joined Villarreal from Man Utd for £2m.He scored 25 goals & won Pichichi Trophy in his first season. 10:41 AM · Aug 21, 2017552329ON THIS DAY: In 2004, Diego Forlan joined Villarreal from Man Utd for £2m.He scored 25 goals & won Pichichi Trophy in his first season. ⚽️ https://t.co/Mit06SZfecForlan only managed 17 goals in 97 games for United. He was sold to Villarreal in 2005, and went on to become an all-time great for Atletico Madrid and Uruguay. The 2010 FIFA World Cup Golden Boot winner had a torrid time in England, though.#4 Memphis Depay - €34 millionManchester United FC vs VfL Wolfsburg - UEFA Champions LeagueManchester United were excited at the arrival of Memphis Depay from the Eredivisie. The Dutch forward was a promising player, but couldn't live up to expectations in English football, though. He scored a paltry seven goals in 53 games for United before joining Lyon 18 months later.Depay had a sensational 2014-15 campaign with PSV, scoring a league-high 22 goals in a title-winning season. Louis van Gaal's United wasted no time finalising a big-money move for him.UtdArena@UtdArenaMemphis Depay's stats for club and country since leaving Manchester United:🎖 143 Games 56 Goals 55 AssistsBeen on since he left the club.3:22 AM · Sep 13, 20191282204Memphis Depay's stats for club and country since leaving Manchester United:🎖 143 Games⚽️ 56 Goals🎯 55 AssistsBeen on 🔥 since he left the club. https://t.co/2PVveV2Gx6However, Depay struggled to adapt to the demands of the English top flight. He produced only 13 goal contributions in over 50 games, which was well below par. After impressive displays for both Lyon and the Netherlands, Depay earned a move to Barcelona last year.