Manchester United and Sir Alex Ferguson were a match made in heaven. The Red Devils earned an enormous amount of success during the reign of the Scottish manager at Old Trafford.Sir Alex Ferguson remains the most successful manager in the history of the Premier League with 13 titles. This speaks volumes of the greatness of the legendary manager.Sir Alex Ferguson leaving Manchester United was heart-breakingThe success that Manchester United saw under Sir Alex Ferguson remains unmatched and therefore it was a very emotional moment when the Scotsman announced his retirement in 2013. Since then, the Red Devils have seen a lot of struggle.Four different managers have managed Manchester United post Sir Alex Ferguson's retirement and yet the Premier League giants haven't won the league once.In the process, there have been some poor transfer decisions made, allowing a number of good players to leave the Red Devils. On that note, let's take a look at those quality players who shouldn't have been sold by Manchester United post Sir Alex Ferguson's era.#5 Wilfried ZahaManchester United v Norwich City - Capital One Cup Fourth RoundWilfried Zaha was signed by Manchester United in the winter of 2013 when Sir Alex Ferguson was still their manager. The deal allowed the Ivorian to return to Crystal Palace for the remainder of the season.The following season. Sir Alex Ferguson retired and his place was taken by David Moyes. Zaha struggled to get regular chances under the former Everton manager, which prompted him to re-join Crystal Palace.HLTCO@HLTCOThis photo is one of my favourite ever Palace images. Wilfried Zaha, looking down at the armband on a day when he scored twice to help Palace beat Manchester United at Old Trafford.8:33 PM · Aug 4, 202182217This photo is one of my favourite ever Palace images. Wilfried Zaha, looking down at the armband on a day when he scored twice to help Palace beat Manchester United at Old Trafford. https://t.co/Nyb0FBwm4OThe winger has amazing dribbling skills, good pace and the ability to take on opponents. A player of Zaha's technical capabilities has been missing from Manchester United for quite some time now. Had he not been sold, maybe things would have been different.#4 Ander HerreraManchester United v Arsenal - Premier LeaguePost Sir Alex Ferguson's retirement, Ander Herrera was one of the most passionate football players that Manchester United had signed. The Red Devils were persistent in their pursuit of his signature and signed him in the summer of 2014.The Spaniard was a very skillful and tenacious midfielder. Herrera is known for his aggression, intelligence and passing skills. His man-of-the-match performance against Chelsea in the Premier League, where he man-marked Eden Hazard, is still very much in the memory of Manchester United fans.Tiny guy★†‡@LesliekatereggaManchester united let ANDER HERRERA go for FREE so as to accommodate FRED. 😂😂😂😂9:45 PM · Nov 20, 20213Manchester united let ANDER HERRERA go for FREE so as to accommodate FRED. 😂😂😂😂Unfortunately, the Red Devils hesitated in offering Ander Herrera a new contract in 2019. He therefore left as a free agent and joined Paris Saint-Germain that summer. A player of his ability is desperately needed at Manchester United right now and it surely wasn't the best call letting him leave.