5 Players Chelsea Should Not Have Let Go in the Premier League Era

It's summer, and as teams jet across the globe on pre-season tours, it's the business of transfers that gets us all excited.

Who is going to be sold? How much money will be spent? Which star will your team chase? It's those questions that help tide fans over before the new season kicks off.

That said, for all the excitement and joy, there is always heartbreak in the air. For every major signing, there are fans somewhere who are disappointed to see a player leave their club.

While Chelsea can boast deep pockets, Blues fans themselves have been on the receiving end of some transfer disappointments in the past. Like any other club, they've had to endure losing some favourites along the way, watching them blossom elsewhere.

So which players do they lament losing the most? Join Bleacher Report as we look at some stars Chelsea should never have parted company with in the Premier League era.

Didier Drogba

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For the fairytale narrative, Didier Drogba's departure from Chelsea in 2012 was perfect. He left Stamford Bridge a king, his last kick of a ball in Chelsea colours coming in the penalty shootout that won the Blues the Champions League in 2012.

In terms of the reality, Chelsea should have fought tooth and nail to keep Drogba. Players of his stature are hard to come by, and despite his powers waning on the pitch, he still had so much to offer in the dressing room.

Much too often, Chelsea have allowed their biggest names to leave because age has dictated it. Without ever replacing their influence, the club has struggled.

On the back of the then-34-year-old Drogba's departure, Chelsea went out of the Champions League in the group stage in 2012/13. They're the only reigning champions to have ever suffered that fate.

Chelsea struggled domestically too, relying on Fernando Torres as their main striker. It proved a painful couple of seasons until they snapped up Diego Costa in the summer of 2014.

By the time he left in 2012, it was right for footballing reasons, but Chelsea forgot Drogba isn't just a footballer; he's a statesman, and the club are lacking them even now.

Graeme Le Saux

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Before John Terry, the big thing coming out of Chelsea's youth system was left-back Graeme Le Saux.

The defender was an exciting player, looking every bit a modern left-back even 20 years ago. Le Saux's athleticism allowed him to bomb up and down the flanks, assisting, preventing and even scoring goals.

He appeared the sort of player Chelsea would build their future around, rare at Stamford Bridge at the time in that he could claim to be bona fide England quality.

He fell out with then-manager Ian Porterfield, and soon after Porterfield's dismissal in early 1993, Le Saux was sold to Blackburn Rovers, enjoying his peak years away from Chelsea. In that time, he won the Premier League with Rovers and became an established international.

Chelsea re-signed him in 1997, and Le Saux played a big part in the success Chelsea would enjoy in the UEFA Cup Winners' Cup, League Cup and FA Cup.

He should never have gone in the first place, though. From selling him for just £700,000, Chelsea paid Blackburn £5 million to bring Le Saux back, ensuring his transfer proved a costly mistake.

Damien Duff

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Selling Damien Duff was the beginning of the end for Jose Mourinho during his first spell as Chelsea boss between 2004 and 2007.

It was by allowing Duff to join Newcastle United in 2006 we saw Mourinho start to play with his so-called more negative tactics—strangling teams rather than blowing them away.

With Duff on one wing and Arjen Robben on the other, Chelsea were irresistible in 2004/05. Not only were they entertaining, but Chelsea were a winning team, and they sealed the title that season in style.

Gradually, though, Mourinho's mindset became about preventing teams from scoring against his side, and playing too expansively was a big risk to that policy. With injuries too, we saw less and less of Duff until he was sold.

Chelsea would still play good football without the Irishman, but within a year, Robben was also gone and the Blues' game was more focused on playing through the middle and squeezing teams.

As we look ahead to 2016/17, manager Antonio Conte's desire to deploy wingers again at Chelsea is bringing about a sense of excitement. When we look back to how Duff and Robben combined, is it any wonder why?

Juan Mata

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Juan Mata only spent two full seasons as a Chelsea player—but what fine seasons they were.

In 2011/12, Chelsea won the Champions League. A year later, they followed up that feat with the Europa League. That victory over Benfica in Amsterdam completed Chelsea's European grand slam, giving them every possible elite trophy in club football.

Mata was a big part of that success, with his performances largely papering over the cracks that were appearing at Stamford Bridge. Despite the trophies, Chelsea finished sixth in the Premier League in 2011/12, while his first two years in west London saw the Spaniard play for four different managers.

Chelsea were transitioning from one generation to the next. Having been named the club's Player of the Year for two seasons running, Mata was supposed to be a big part of that. In 2014, though, he was sold to Manchester United, with Mourinho not playing him and cashing in on his value.

For a club such as Chelsea, it's difficult to suggest they have been missing Mata. They have other star names, such as Eden Hazard and Cesc Fabregas, who have produced in his absence. That said, Chelsea have lacked a player of his craft in central areas, and after everything he did in Chelsea colours, there's every reason to suggest he would have been a big figure still.

Kevin De Bruyne

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Having produced very little in a Chelsea shirt, it's not hard to see why the Blues allowed Kevin De Bruyne to leave when he did.

Back in January 2014, Chelsea sold De Bruyne for £18 million—a sum that gave them an £11 million profit on a player who hadn't made appearances in the double figures for them.

It was what De Bruyne had produced while on loan with Werder Bremen that had got everyone excited. While playing the Bundesliga, De Bruyne had been a standout attraction, and Chelsea fans hoped he would repeat that at Stamford Bridge.

For whatever reason, he struggled under Mourinho, and within six months, the Belgian was being sent back to Germany, but this time it was a permanent move to Wolfsburg.

In his 18 months there, De Bruyne became an even better than we had seen in Bremen, winning Germany's Footballer of the Year award for 2014/15. That convinced Manchester City to pay £55 million for him last summer.

He's lived up to that price tag too, being one of the club's best performers since. Indeed, it was his form that took City to last season's Champions League semi-finals.

Had he been given time at Chelsea, there's no telling what the club would have on their hands.

Garry Hayes is Bleacher Report's lead Chelsea correspondent. All quotes were obtained firsthand unless otherwise noted. Follow him on Twitter @garryhayes.