Regret. In the world of football, it is a term that quite often comes to haunt clubs, big or small. It is a sentiment that quite often manifests whenever clubs miss out on key signings, or when clubs let go of current players. It is these very players who go on to do bigger things in their careers than what the club had initially expected of them. It is a fact that clubs, in order to raise capital, do see selling their current players as a viable business model. Clubs such as Borussia Dortmund and Chelsea have even made it the lifeline of their finances. Buying young talent for a relatively cheap price. Later, developing them and selling them for a profit. However, a club like Manchester United does not find itself in that bracket. As the club's immense commercial success off the field is healthy enough to balance the books and finance high-profile acquisitions.Now, although Manchester United is not traditionally regarded as a selling club. They do have a history of trades which have surely brought them some regret over the years. As it is these very players who have gone on to achieve glory and more for the clubs they have gone on to play for. On that note, let's take a look atTop 5 players Manchester United regret losing#5 Diego ForlanManchester United fans tend to have a strong memory. Whenever Diego Forlan's name comes up, it is sure to leave a smile on the faces of those who remember the time when he scored a screamer and ripped off his jersey while celebrating. Yet one of his more memorable moments at Manchester United will surely be the goals he scored against Liverpool. It was a game where he won the hearts of those at the Stretford End, as they sang:"He came from Uruguay, he made the Scousers cry!"Diego Forlan, a Manchester United signing from 2002, is surely a player United have regretted letting go of. During his spell, Forlan found himself third in the pecking order after Ruud van Nistelrooy and Ole Gunnar Solskjær. Following this, he was deemed surplus to requirements and was sold to Villareal. The following season, Diego Forlan went on to win the 2004-05 Pichichi award and the European Golden Boot. A signing United will surely regret letting go of.Warriors of Uruguay@UruguayHeroesDiego Forlán - Pichichi winner in 2005 and 2009 - has named 5 players as his favourites to be crowned topscorer of Spain this season.🇫🇷| Karim Benzema🇫🇷| Antoine Griezmann🇺🇾| Luis Suárez 🇪🇸| Gerard Moreno 🇳🇱| Memphis DepayWho are you backing to become Pichichi?10:50 PM · Sep 8, 2021832Diego Forlán - Pichichi winner in 2005 and 2009 - has named 5 players as his favourites to be crowned topscorer of Spain this season.🇫🇷| Karim Benzema🇫🇷| Antoine Griezmann🇺🇾| Luis Suárez 🇪🇸| Gerard Moreno 🇳🇱| Memphis DepayWho are you backing to become Pichichi? https://t.co/Qn2qIcuOrH#4 Romelu LukakuManchester United v Liverpool - Premier LeagueRomelu Lukaku's string of splendid performances at Everton meant that in 2017 he would sign as the new number nine for Jose Mourinho's Manchester United. Although Lukaku got off to a flying start during his first season at the club, his time at United could be best described as an unwanted striker after Mourinho's spell ended the following season, and in came Ole Gunnar Solskjær. Lukaku no longer fit into Solskjær's tactics as he demanded a highly mobile, pressing, and lethal forward.However, after having moved on to Inter Milan in 2019, Romelu Lukaku proved everyone wrong. As it was his performances with 30 goals during the season that made sure Inter Milan won their 19th Scudetto.Romelu Lukaku is back with Chelsea in the Premier League and is sure to add to the regrets of Manchester United.UEFA Champions League@ChampionsLeague🔵 Romelu Lukaku has now scored 4 goals in 4 games for Chelsea. Signing of the season? #UCL02:09 AM · Sep 15, 2021238752533🔵 Romelu Lukaku has now scored 4 goals in 4 games for Chelsea. Signing of the season? #UCL https://t.co/1fdMo2fZTI