Why Diego Costa Is the 1 Player Chelsea Can't Afford to Lose in the Summer

Diego Costa's goal return since the departure of Jose Mourinho outlines just how vital the Spain international is to Chelsea.

Of the top five deadliest goalscorers in English football, only Harry Kane and Sergio Aguero have outscored the Stamford Bridge forward since Boxing Day.

The England international has 16 goals, with the Argentinian one ahead on 17 in all competitions. Costa sits third in the list of goalscorers with 11.

Those figures for the same period put him above Jamie Vardy, Romelu Lukaku and PFA Player of the Year Riyad Mahrez, who complete the Premier League's top five for goals scored over the course of 2015/16.

Had it not been for Chelsea's woeful first half of the campaign, we should assume Costa would be right in there among the leaders, as he was last year. It's for that reason alone the Blues can't afford to lose him this summer.

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Antonio Conte has a big job to restore the fear factor at Stamford Bridge. Too often we've seen opponents face Chelsea without much concern and come away with their reward. Crystal Palace, Southampton, Liverpool, Bournemouth and Manchester City have all stepped into the lions' den and realised the roar isn't backed up by anything of substance.

Conte will want to add some mettle in midfield for his side to regain that physical and psychological edge. It'll be similar at the back, but Chelsea need goals more than anything to strike at the heart of opponents. Costa is the goalscorer to do that, as he has proved since Christmas.

We can only speculate as to how much of an influence Mourinho had on his barren spell in front of goal earlier this term. What's clear, though, is that the slight changes Guus Hiddink has made have brought about Costa's return to form.

He was never the problem for Chelsea when things turned sour under Mourinho. Opponents stuck to the basic principle that, to stop a goalscorer, it's cutting off the supply line that is most effective. When Cesc Fabregas was being charged in midfield and Eden Hazard was off form, Costa was living off scraps.

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Since Fabregas has got back to impacting games, Costa's been scoring regularly again.

Indeed, we can point to goals scored for a hint of that, but then digging a little deeper with Aguero and Kane, we see that Costa's strike rate since Christmas is a better indicator.

He's averaging 0.57 goals per game compared to 0.64 Kane's and Aguero's 0.60. The pair remain more lethal, yet for a player in a struggling side, Costa has kept up a healthy return that rivals them.

Comparing him to those leaders in the Premier League is important, as it gauges where he is right now. For Chelsea to be back challenging for major honours next season, it's those sort of players Conte's men will need to usurp.

Where most of this current Chelsea squad are failing, Costa is succeeding. He's showing his ability among the Premier League's finest, outdoing others like Vardy and Mahrez who have hit a similar wall to the one the former Atletico Madrid man struck in the early part of the season.

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The frustrating thing for Chelsea is that collectively their season hasn't mirrored the swing in Costa's form. Whereas the striker got his numbers up, too many others in the squad haven't improved enough.

Chelsea still have major concerns defensively, and that weak back line has proved to be their Achilles' heel. With four games still to play, they've conceded 46 goals in the league, which equals their 2011/12 total as their worst defensive record since 1996/97.

It's those areas that will be of most concern for Conte in the immediate future. Fortunately for him, he has a striker scoring goals at the other end.

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