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Some clubs have an awful habit of letting talent slip through their fingers.
It must be so frustrating for fans to see a potential superstar allowed to leave for very little money, only to go on to excel at a different team.
After Jerome Boateng recently appeared on
Jack Whitehall: Training Days, we couldn't help but wonder how much Manchester City fans resented the fact that their former player has gone on to become one of Europe's top defenders.
Boateng is not alone, however. Several players have flourished after being allowed to leave for greener pastures.
Here are just seven names that spring to mind.
Dennis Bergkamp - Inter Milan
Inter Milan's decision to sell Dennis Bergkamp for £7.5 million in 1995 was not exactly surprising at the time as the Dutchman had struggled to fulfil his potential at the San Siro.
Having been reduced to a punchline when one Italian outlet renamed their
L'asino della settimana (Donkey of the Week) award the
Bergkamp della settimana (Bergkamp of the Week)
, the forward went on to become a revelation at Arsenal.
His goals feature regularly on highlight packages of the best ever in the Premier League and he will go down as one of the greatest ever Gunners thanks to his 120 goals for the North London side.
Roberto Carlos - Inter Milan
Arguably the greatest left-back of all time has blamed poor ol' Roy Hodgson for his shortcomings at Inter Milan.
Brazilian legend Roberto Carlos has claimed that Hodgson's decision to play him in midfield on several occasions hindered his chances to succeed in Italy.
"It's not that I did not have a good relationship with Hodgson," said Carlos. "It is just that Hodgson doesn't know much about football."
Luckily for Carlos, his move to Real Madrid worked out rather well...
Jerome Boateng - Manchester City
Defenders don't often get the credit they deserve but Manchester City supporters will be fully aware that they didn't see the best of Jerome Boateng.
The centre-half played just one season at the Etihad before heading to Bayern Munich, where Boateng has won six consecutive Bundesliga titles.
He's also managed to pull off the little feat of winning a World Cup and continues to prove that he's one of the most reliable defenders in the world.
Paul Pogba - Manchester United
Just from a financial standpoint, Manchester United must really regret letting Paul Pogba move to Juventus in 2012.
Pogba became a world-class talent after deciding against signing a new contract with United six years ago and the Red Devils clearly acknowledged the error of their ways by shelling out £89.3 million to get him back in 2016.
His return to Old Trafford has not exactly been spectacular and while he has yet to justify his massive price-tag, he's definitely proven that United were foolish to let him leave in the first place.
Gerard Pique - Manchester United
You can't argue with results.
Since Pique left Manchester United for £5 million, the central defender has lifted the World Cup and has also won three Champions League titles, seven La Liga championships and six Copa del Reys.
That's a lot!
Kevin De Bruyne - Chelsea
Chelsea, here we are again!
Jose Mourinho claimed that Kevin De Bruyne was "crying every day he wants to leave" when he wasn't receiving the game time he desired and the then-Blues boss made the decision to sign off on a deal which saw the Belgian midfielder leave for £18 million.
Now, in 2018, he's perhaps the best playmaker in England and Pep Guardiola's faith in him is paying off like Mourinho wouldn't believe.
Mohamed Salah - Chelsea
From Chelsea reject to legitimate Ballon d'Or contender in just a couple of short seasons.
Only Lionel Messi scored more league goals than Mohamed Salah last season and the Egyptian forward is suddenly being valued at over £200 million, after being allowed to leave Stamford Bridge for £13 million three years ago.
Liverpool supporters likely wouldn't have believed you if, a year ago, you told them that their team would be contesting the Champions League final with Real Madrid but Salah has come along and changed the entire football club.
Chelsea fans must be raging...
Watch more of Jack Whitehall: Training Days on YouTube. 
