Why It's Vital for Borussia Dortmund to Keep Hold of Mats Hummels

Looking at the contract situations of a number of important players at Borussia Dortmund, the initial instinct could be that, if anyone, Mats Hummels is the most expendable.

The 27-year-old is one of four regular starters whose contracts expire in 2017.

Left-backs are hard to find, so the club need to keep a hold of Marcel Schmelzer. Ilkay Gundogan is the midfield mastermind and would leave a yawning gap behind if he were to move abroad. Henrikh Mkhitaryan is arguably the club's player of the season, and his 20 goals and 28 assists across competitions, per Transfermarkt.co.uk, would be very difficult to replace.

Centre-backs, however, usually aren't too expensive and, unlike those other positions, the Black and Yellows have some in-house candidates to fill a more prominent role in the heart of defence.

If truth be told, though, Hummels isn't an ordinary centre-back. One could argue he's the least expendable player Dortmund have in their ranks. Having rediscovered his form after a slew of disappointing performances in autumn, the Germany international has been the Ruhr side's most consistent player since the turn of the year.

On his game, Hummels transcends his position. He's a creative midfielder in a centre-back's body, as this stat alludes to:

With that come a few weaknesses in defence—he tends to misjudge balls over the top, for example, as seen against Bayern Munich and Liverpool this season, and against the Republic of Ireland on international duty—but head coach Thomas Tuchel gladly hazards the consequences of Hummels' forward-thinking approach.

His impressive qualities on the ball are well worth the occasional defensive mishap. How many central defenders would even try a cross like this one against Werder Bremen?:

"

El gol de Mkhitaryan para que el Dortmund tome la ventaja antes de ir al descanso. pic.twitter.com/8RkVrZReSn https://t.co/tqb4O7aKm8

— LOCO X EL FÚTBOL® (@LOCOXELFUTTBOL) October 31, 2015"

As the compilation put together by Daniel Gubala on Vimeo shows, that amazing assist was no outlier. Hummels shows almost surgical precision in build-up play and has been a major part in Dortmund's revival during Tuchel's first year at the club:

Hummels' importance to BVB extends beyond the playing field: Captain since 2014, he's the unquestioned leader of the team.  The only "real" 2014 FIFA World Champion in the squad—no other Dortmund player took to the field in Brazil—Hummels is a face of the club.

Outspoken and intelligent in interviews, he served as the team's mouthpiece even before he got the captain's armband.

One of his more recent interviews gave pause, however. Speaking to German tabloid Bild, the 27-year-old gave an insight into his thought process during the contract talks (h/t the Daily Star's Marc Williams):

"

I do not necessarily want [to leave], but I have to make a big decision. I believe that all parties want to clearly hear the decision before the European Championship. 

Now, if I do sign a long-term contract [with Dortmund], I will be 31 or 32 years old at the end of the season. At the moment I do not plan to play until I'm 38. 

I want to play a certain kind of football but I also want to win titles.

The better the team, the greater the stimulus, to be part of it. This is not an easy decision and I have been back and forth. I hope that there is at some point a clear direction.

"

Hummels has to make a fundamental decision in the next few weeks. At the club since January 2008, an extension would likely mean he'd spend his entire career at the professional level at Signal Iduna Park, at least his entire prime.

Alex Livesey/Getty Images

In that same interview with Bild, Hummels made one statement that could be interpreted as a sign he'll leave Dortmund (via Stefan Buczko of YellowWallPod.com): "I realise in every year I don’t win a title, I’m closer to retiring. The [UEFA] Champions League is a target for me in any case. I want to win this title."

The Black and Yellows' impressive season notwithstanding, Dortmund will always be an outside bet for European football's biggest prize because of their lack of financial firepower in comparison to the big guns.

If one of those comes calling, Hummels could leave the club. It would leave a gaping hole.

Lars Pollmann is a Featured Columnist writing on Borussia Dortmund. He also writes for YellowWallPod.com. You can follow him on Twitter.