Diego Costa's Fitness Vital for Chelsea During Busy Christmas Schedule

Now the Premier League is getting exciting.

Christmas is here and the title race is threatening to take off. Manchester City have drawn level on points with Chelsea after their 3-0 defeat of Crystal Palace on Saturday, and the pressure is on for Jose Mourinho's team to react.

With their game in hand against Stoke City on Monday night, Chelsea need a win to start their festive campaign in the best possible way.

To do that, they need Diego Costa firing like he was earlier in the campaign.

Injuries and—to a lesser degree—suspensions have forced the Spanish international to endure a stop-start campaign.

When he's been on form, he's been formidable, but after scoring nine goals in the first two months of the season, Costa has managed just three since the beginning of October.

Mourinho has guarded his prized asset, resting him when needed, unleashing him on opponents at the right moment.

It's no coincidence that his absence and lack of form in recent weeks has coincided with City's resurgence, however.

While Costa wasn't scoring and Chelsea were dropping points against Sunderland and Newcastle United not so long ago, Manuel Pellegrini's City were quietly gathering momentum. They now have the bit between their teeth.

City aren't going away, and the only way to keep their threat at arm's length is by Chelsea having their best players on form.

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Cesc Fabregas and Eden Hazard provide the flair in this team, Costa the goals. It needs to be that way over Christmas.

This will be the latter's first experience of the congested winter schedule in England. In Spain, he was used to a period of rest, but the Premier League doesn't offer players such luxuries.

It's grueling. In just 11 days, Chelsea will play four games. With two of them being London derbies and another the cliched trip to Stoke's Britannia Stadium on a cold Monday night, it's going to be tough.

Over the next week or so we're going to learn plenty about the strength of Chelsea's title challenge: Can they go the distance? Do they have the character? Will they crumble under the pressure?

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Another burning question will be how Costa reacts. How will his hamstrings hold up? Is he ready to play games back-to-back?

Should the answer be "yes" to the latter of those, things will be looking positive for Chelsea and their leading goalscorer.

Should it go the other way, the Premier League pendulum could very well swing in City's favour. With so many games in a short space of time, it isn't beyond the realm of possibility that they can be out of sight before the year is out.

Now's not the time to be losing form or getting injured.

Aside from Sergio Aguero, there isn't another striker in the Premier League that puts the skids up defenders like Costa does.

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His presence has restored the fear factor whenever Chelsea come to town. Costa has transformed them into a juggernaut—the winning machine Mourinho has craved.

Without him, Mourinho's side lack bite. He brings a constant threat.

Didier Drogba has rolled back the years since his return, but Costa's a different breed of striker. Whereas Drogba has to pick and choose his battles more strategically in games these days, Costa is a 90-minute man.

Come Monday, Ryan Shawcross will know he's been in a game after the final whistle. The Stoke defender would have been physically challenged for the duration, forced to track Costa's constant runs off the ball.

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His legs will be tired, his mind even more so.

Costa's unrelenting, a player who keeps knocking on the door until he eventually kicks it down. That's the edge every team needs at this time of year.

Now that City have shown they're for real, Chelsea have to react.

Costa was signed by Mourinho to deliver the title. If he remains fit over Christmas, he may well do just that.

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