Chelsea Cruise into Champions League Last 16, but Diego Costa Draws Blanks Again

LONDON — It was supposed to be 'academy day' at Stamford Bridge when Chelsea faced Sporting Lisbon in their final group game of the Champions League.

Instead, it turned into another nightmare for Diego Costa. Well, in front of goal at least.

Heading into this game, manager Jose Mourinho had sung the praises of Chelsea's youth system, and with so many star names rested, he had hinted that he would field a team complete of youngsters.

Instead we saw Chelsea's B-team, as the likes of Mohamed Salah and others who are searching for fitness were given a run out.

The only glimpse of youth on display was the introduction of 18-year-old Ruben Loftus-Cheek, replacing Cesc Fabregas on 83 minutes to make his Chelsea debut.

Chelsea's place at the top of Group G was already guaranteed, so from their perspective, the game was a dead-rubber.

Whereas Andre Schurrle and Kurt Zouma impressed with rare starts, however, Costa's frustrating form continued.

Since returning from injury, the Spanish international hasn't looked the same player who started the campaign in such fine goalscoring form. He drew another blank against a poor Sporting outfit.

Paul Gilham/Getty Images

Indeed, with their Champions League future on the line, Sporting offered very little. At times, it looked like Chelsea were the side battling to stay in the competition, outplaying the Portuguese outfit while hardly breaking a sweat.

As for Costa, it's not a crisis just yet, but that's four games now since he last found the back of the net in November's 2-0 win against West Bromwich Albion.

Before this run of poor form in front of goal, Costa's performances were such that he had only failed to score in five Chelsea games all season—two of which he had started as a substitute.

This run has already seen him close to surpassing that record, and it must be a concern.

Rather than rest his star man on Wednesday, Mourinho knew Costa needed game time to rediscover his scoring touch, but conceded there is work to do.

"The confidence is not high, the condition is not the best," he said of Chelsea's leading goalscorer, but added he was encouraged by his performance.

"I saw him better than Newcastle, better than against Sunderland."

Mourinho has a point. Had it not been for some last-ditch defending, Costa could have come away with a hat-trick at Stamford Bridge and humbled Sporting further.

Paul Gilham/Getty Images

He was making the right runs, finding space at the vital moments. Yet there was a difference: Unlike before, he wasn't punishing the opponent.

And he hasn't been for some time.

That's the root of the problem for Costa. Right now, he isn't the force we know he can be, and defenders are getting the better of him in ways we haven't seen since he moved to England.

From ruthlessly capitalising on the slightest error or defensive lapse, he's letting opponents back in to make amends.

Clive Mason/Getty Images

Against Sporting, it was a heavy touch when through on goal. Against Newcastle United last weekend, it was a lack of conviction in the box. Previous matches have seen other instances when Costa should have done better.

Optimists will suggest it's all a blip for Costa, a result of a stop-start campaign catching up with him. Pessimists will point to a more deep-rooted problem, that Costa's ongoing fitness issues are restricting his impact.

It's probably a mixture of both. He defied the odds when struggling with his hamstring woes, proving to be as elusive as ever, but how long did we really think it would continue?

Costa was so effective that he's become a victim of his own success—we expect him to score in every game he plays regardless of his health.

Paul Gilham/Getty Images

We see Costa, and visions of what he has achieved in his short Chelsea career blur the vision when the reality is he is still in recovery.

Teams have worked him out, though, and his readjustment into this Chelsea side needs to pick up pace. Quickly.

Costa's not in the midst of a drought just yet, but how long can his lack of goals continue? Given the recent history the Blues have endured with strikers, Mourinho would be lying if he suggested he wasn't slightly concerned with the situation.

The fixtures are piling up. Chelsea need Costa back. Now.

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