We've seen the best of Willian in a Chelsea shirt of late. Now Diego Costa needs to step up.The Chelsea striker hasn't looked his rampaging best this season.Indeed, since he returned from a three-game suspension for his part in the shenanigans that saw Arsenal's Gabriel Paulista sent off at Stamford Bridge in September, Costa's lacked the same edge that's so endeared him to the Chelsea crowd.Costa came in for plenty of criticism during Chelsea's 2-0 defeat of the Gunners, but it was also probably his best display this term.The Spanish international gave the Arsenal defence a going over that dayalthough he did over step the mark at times. But seldom will Laurent Koscielny and the rest of his teammates face such a stern test, physically and technically.Costa's warrior spirit against Arsenal is what ultimately gave Chelsea a rare three points this season; it's what sets him apart from other strikers.He plays the game to the limits, battling for every yard as he works tirelessly for his team. He'll run the channels, drop deep and do his bit defensively when needed.It's not just about goals, and Costa knows it.When teams are stuck in the sort of rut Chelsea find themselves, it's that sort of application and attitude that goes a long way toward reversing their fortunes.It's the players with character who inspire the rest of those around them.John Terry led by example with the way he addressed the media in his press conference this week, while Willian has more than played his part with his endeavour.Willian's form has been infectious. He's been the player those around him have looked to and has drawn a response from his team-mates.Wednesday's Champions League victory over Dynamo Kiev was largely down to his brilliance. Willian assisted the opening goal and scored the winner with his sublime free-kick.That was a moment of inspired genius when Chelsea needed it most. Kiev had not long equalised, and Chelsea's heads were dropping in the same way we've seen in every game in which they've been pegged back this season.It happened most recently against Liverpool when they failed to respond, eventually losing 3-1.Without Willian, it may well have happened again against Kiev.It's why Costa must rediscover that fighting spirit for which we know him. He needs to help keep those chins up when leading the line.Willian and Terry can't do it all alone.Chelsea can ill afford to be so fragile. This is a team of champions, but they haven't looked like it since August.Sitting 15th in the Premier League table, Chelsea are where they deserve to be, given how opposing sides have made mincemeat of them.They've lacked confidence; they've lacked character; they've lacked warriors.In the Chelsea dressing room, few embody the description of a warrior better than Costa.He's not a product of a youth academy; he's come from the streets and had to fight his way to the very top.His story is one to be marvelled at, one defined by that warrior ideal.It's why his performances of late have been below par. He has reined in that side of his game too much. Now he must find a balance.When Costa is scratching his opponents or leaving a flying boot lingering too long, he oversteps the mark and deserves any punishments that are dished out.Yet when he's playing with a smile, he's equally as much use to Chelsea when watching from the stands.He's the sort of player who thrives on the conflict, channeling his aggression to punish teams. He's a back-to-the-wall sort who needs that conflict to drive him on.In many ways, it's what we've grown used to seeing from Chelsea in this past decade or so. Despite the billions Roman Abramovich has invested, it's always been that fighting spirit which has earned the club respect.Chelsea have never hit the same heights as Barcelona when we consider their footballing philosophy, but they have won things their own way through mixing their talent with graft.They've had to fight plenty for it, too.In the past, it was Didier Drogba as the striker delivering on that mandate, but now Costa must continue that tradition.For too long Chelsea lacked any real presence without Drogba, and it cost them in the silverware stakes. They simply weren't a big enough threat in front of goal.Chelsea didn't scare teams without Drogba; if his own personal slump continues, Chelsea aren't going to strike fear in teams with Costa leading the line, either.With Costa at his nasty best, Chelsea will win matches. Winning matches will see them climb the table in the short term, which will return the club's confidence to where it needs to be.Before any of that can happen, Costa must rediscover his inner warrior. Without that he's toast, and so are Chelsea.Garry Hayes is Bleacher Report's lead Chelsea correspondent. All quotes were obtained firsthand unless otherwise noted. Follow him on Twitter @garryhayes
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