Regrets, they've had a few, writes Greg Lea – and some clubs too many to mention...
1. Pierre-Emerick Aubameyang
The Bundesliga’s top scorer has been linked with the likes of Barcelona and Real Madrid in recent months, with 29 goals in all competitions for Borussia Dortmund this season making him one of the hottest marksmen on the planet.
Milan paid £21m for a prolific centre-forward of their own in Carlos Bacca last summer, money they could have saved had they simply kept hold of Aubameyang, who was sold to Saint-Etienne in 2012 after a grand total of zero appearances during a five-year spell at San Siro in which he spent time on loan at Dijon, Lille, Monaco and les Verts.
2. Jerome Boateng
The then 22-year-old played only 16 times under Roberto Mancini in the Premier League after joining from Hamburg 12 months before his departure
As Manchester City’s defensive frailties threaten to undermine their push for the Premier League title, the club’s supporters may well continue to lament the decision to sell Boateng – arguably the best centre-back in the world right now – to Bayern Munich in 2011. The then-22-year-old played only 16 times under Roberto Mancini in the Premier League after joining from Hamburg 12 months before his departure, with Bayern able to bring him back to Germany for just €13.5m. Three Bundesliga titles and a Champions League crown later...
3. Giorgio Chiellini
Chiellini has been a key part of the Juventus team that has dominated Serie A for most of this decade, but things could have been so different had Roma not sold their share in the then-19-year-old to Livorno in 2004. Two years before that, the Giallorossi had purchased half of his rights with a view to acquiring him outright at a later date; instead, they allowed Livorno to pay €3m to secure 100% of Chiellini’s contract, with the Tuscans then selling him on to Juve in 2005.
Chiellini nets against the former club he never played for
4. Paul Pogba
That was one of only seven outings in a United shirt, with Pogba’s frustration at the lack of opportunities culminating in him joining Juventus when his contract expired at the end of that season
On balance, Sir Alex Ferguson got more things right than wrong during his time as manager of Manchester United, but his handling of Pogba must go down as one of the biggest mistakes of his 26-year career at Old Trafford. The Frenchman joined United from Le Havre as a 16-year-old in 2009, before making his first-team debut in a League Cup tie with Leeds United in the 2011/12 campaign.
That was one of only seven outings in a United shirt, and Pogba’s frustration at the lack of opportunities culminated in him joining Juventus when his contract expired at the end of that season. He hasn’t looked back since, winning three Serie A titles, reaching the final of the Champions League and becoming the most in-demand young player around.