Was Selling Ryan Bertrand Among Jose Mourinho's Biggest Errors as Chelsea Boss?

When Chelsea were swept aside with merciless ease at home to Manchester City in April, it was the closing chapter to end the debate over the wisdom in the club selling Kevin De Bruyne when they did.

The Belgian had arrived at City in 2015 after an 18-month stint with Wolfsburg, who had signed him from Chelsea in January 2014 after he was deemed surplus to requirements by Jose Mourinho. The Bundesliga side made a £37 million profit on the sale of De Bruyne, with City's £55 million fee far outstripping the £18 million Wolfsburg had paid for him.

That season-and-a-half spell back in Germany saw De Bruyne's stock rise significantly, yet there was still a lingering doubt about his suitability to Chelsea. Regardless of what City were paying for him, Chelsea supporters and, indeed, Mourinho weren't totally convinced he was worth it.

If seeing is believing, De Bruyne made all inside Stamford Bridge stand up and take notice in April. Mourinho may have been gone from the club himself by that stage—he was sacked five months earlier—but De Bruyne's performance at the Bridge was so exceptional, Mourinho wasn't important. He ripped through the Chelsea midfield at will and was an influential figure in a 3-0 victory.

It was the final confirmation of what we all knew; a rejected player returning to the venue where it had all gone wrong for him personally, driving a final nail in the coffin of a club's season and potentially the era that had delivered so much for Chelsea.

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Watching De Bruyne, we saw exactly what Chelsea weren't: exciting. His desire to play the game direct and at pace was the antithesis of what Chelsea had become or were. They were champions, yet they couldn't have been further from such status. They were laborious; they looked old, uninterested.

They had to change; Chelsea had to move with the times, otherwise they would be forgotten.

That the decline had come so rapidly shocked many. How could a team so dominant in the Premier League just 12 months earlier appear so weak? How could Chelsea have gone from champions to being so insignificant in just one season?

The truth is that it had been coming. The club had done well to paper over the cracks in years gone by, although constant managerial changes and ill-judged transfers had created a melting pot of failure that was ready to boil over. In 2015/16, it did.

Another symbol of Chelsea's flawed policies—Mourinho's included—is Ryan Bertrand. The Blues will face Southampton this weekend, and while Bertrand may well miss out against his former club through a hamstring injury, his emergence on the south coast serves to frustrate.

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Bertrand is a Chelsea academy product, and a big part of his development was stunted by him playing in the same position as Ashley Cole. For all his talents at left-back, Bertrand's early-20s meant he had to travel for his football as he was up against Cole in the first-team stakes—a player widely regarded as the best in the world in his position during his pomp.

That was understandable as the aim was never that Bertrand would be usurping Cole when he was 21. He was supposed to do that when he was in his mid-20s, like he is now. Chelsea's plan seemed to be much more long-term; Bertrand was supposed to be Cole's long-term replacement.

So when the latter's legs eventually gave up on him in Mourinho's first season back at Stamford Bridge in 2013/14, the expectation was that Bertrand would finally get his chance. He didn't; a right-back became Chelsea's left-back.

Indeed, until Antonio Conte's new system at Chelsea this term, Cesar Azpilicueta has been the first-choice left-back for three seasons. A temporary solution maybe, playing Azplicueta out of position for such a prolonged spell was never going to be of benefit to the club or the player.

In the middle of all this, Chelsea sold Bertrand to Southampton for £10 million in January 2015. Despite all of Cole's problems and no other natural fit for his position, Bertrand played just three games under Mourinho in 18 months. He wasn't given a chance.

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Now we see him flourishing at St Mary's, like he did last season when Chelsea's problems really hit home. With Branislav Ivanovic suffering a similar decline to Cole's, Chelsea didn't have the players to arrest it. Azpilicueta was busy on the left, and it meant the team had to persevere.

In the time since Cole left Chelsea, the club has made a net spend of £45 million on left-backs. They've bought and sold Filipe Luis from and to Atletico Madrid for £16 million; Baba Rahman was signed for £22 million last season, while Marcos Alonso was added for £23 million on transfer-deadline day just past.

For all those fees and the effort in getting the deals done, why couldn't Chelsea have shown more faith in Bertrand? Why did Mourinho not give him his chance to become established at Stamford Bridge? It was always the plan that wasn't carried through.

Bertrand is an international quality left-back with 10 England caps—some of which were won while at Chelsea. When we consider how Conte's got his Chelsea team playing, it's evident he is of a standard to at least compete in this side.

With his attacking ability, Bertrand would suit the wing-back role Alonso is playing now; in the final third, he would be a threat to complement Eden Hazard, Diego Costa and Victor Moses on the opposite flank.

Chelsea have had to spend big money to replace Cole when the answer was staring them in the face through Bertrand. It's here where Mourinho really failed Chelsea.

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After last week's 4-0 thrashing of Mourinho's Manchester United, Chelsea fans can start to properly move on from his reign. He has lurked in the background for much too long in west London, haunting the club with what he achieved and the silverware that he stocked in the trophy cabinet. That defeat seemed to cut the umbilical chord once and for all.

A desire to spend and not promote from within has hurt the club. Even youngsters not part of the academy, but signed as part of Chelsea's redevelopment, weren't sufficiently used. We can point to Romelu Lukaku as another high-profile name in that list, adding him to De Bruyne.

We shouldn't forget Bertrand in all that. Quiet and without much of a profile away from the game, he slips under the radar somewhat. Watching him in Southampton colours now, though, there will be coaches in the junior ranks at Stamford Bridge wondering why he was never allowed to excel at the club.

Had he done so, Chelsea's bank balance would be looking far healthier. Perhaps those problems from last season wouldn't have been as damaging as they would prove.

Mourinho's Chelsea legacy now seems to be that we're left talking about his mistakes, not his victories. Bertrand is a big one of them.

Garry Hayes is Bleacher Report's lead Chelsea correspondent. Follow him on Twitter @garryhayes