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.
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.
5. Samuel Eto’o
He pulled on the famous all-white kit on only six occasions, however, and was sold to Mallorca in 2000
Luis Figo, Michael Laudrup, Luis Enrique and Ronaldo are the players who crop up most often in discussions of those few who represented Real Madrid and Barcelona, but Eto’o is another big-name star who experienced life on both sides of the Clasico divide.
The Cameroonian, who joined Madrid’s youth academy aged 16 in 1996, spent time on loan at Leganes, Espanyol and Mallorca during his four years on the capital club's books. He pulled on the famous all-white kit only six times, however, and was sold to Mallorca in 2000.
It wasn't Madrid’s finest hour: now 34, Eto’o has enjoyed a stellar career since leaving the Bernabeu, with four league titles and three Champions Leagues won at Barça and Inter.
6. Mats Hummels
Bayern Munich’s squad features a handful of players who have came through their youth system, with Philipp Lahm, Thomas Muller and Holger Badstuber providing a homegrown core to the Bundesliga champions’ makeup. Hummels may currently ply his trade with Borussia Dortmund but he too is a Bayern academy graduate, having first joined the club as a six-year-old in 1995.
The central defender only ever played twice for the Bavarians before moving to Dortmund on a permanent deal in 2008; while it’s difficult to assert that a team who have conceded just 14 goals all season could do with him in their side today, it’s clear that the 44-time Germany international has exceeded Bayern’s expectations in the eight years since he left.
7. Andrea Pirlo
Juventus have been good at this game down the years. When Milan let Pirlo join the Old Lady on a free transfer in 2011, they became the second club from Italy’s second city to allow the midfield metronome to exit prematurely.
It was a decade previously that Inter sanctioned the 22-year-old Pirlo’s departure from San Siro for the first time, the future World Cup winner deemed surplus to requirements and permitted to join the Nerazzurri’s dreaded rivals.
Pirlo, who turned out 22 times for Inter in Serie A, went on to claim two league titles, two Champions Leagues and a Coppa Italia with Milan, before they too made the same mistake four-and-a-half years ago. No Pirlo...
8. Gerard Pique
Manchester United is a great club and I enjoyed winning things. To play with world-class players has helped me and now I want to continue this at Barca
“I am very happy to be back,” Pique told the Spanish press after re-joining boyhood club Barcelona seven-and-a-half years ago. “I didn't think I would be here again but this is very good for me. Manchester United is a great club and I enjoyed winning things. To play with world-class players has helped me and now I want to continue this at Barca.”
Pique has had few problems doing just that after struggling to displace Rio Ferdinand or Nemanja Vidic at Old Trafford between 2004 and 2008, a spell in which he was limited to just six Premier League starts.
9. Thierry Henry
Having paid Monaco £10.5m for Henry in January 1999, it surely would have been worthwhile for Juventus to persevere with him for a little longer than seven months.
In fairness, though, there was little in Henry’s performances for the Old Lady to suggest he would make it at the highest level: the future Arsenal man mustered just three goals in 19 appearances and simply looked out of his depth at the top end of Serie A.
Juve’s loss was the Gunners’ gain, with Henry moving to Highbury in June and becoming one of Europe’s most dangerous strikers soon after.
Cristiano Ronaldo may have overtaken Raul to become Madrid’s record goal-getter last year, but that is frankly scant consolation for followers of Atleti
There can't be many things more painful to a football fan than knowing your club effectively supplied your biggest rivals with the player who would go on to become their all-time record goalscorer.
That, though, is exactly what happened to Atletico Madrid in the early 1990s, when the financially motivated decision to shut down the club’s academy meant Raul hopped across town to sign for Real Madrid aged 17. The deadly frontman found the back of the net 323 times before leaving in 2010, helping los Blancos to nine major honours and establishing himself as one of the greatest players in their history.
Cristiano Ronaldo may have overtaken Raul to become Madrid’s record goal-getter last year, but that is frankly scant consolation for Atleti followers.
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