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"Form is temporary; class is permanent." A cliche, of course, but one that, at its core, is probably true. When form is permanently bad, however, class on paper doesn't matter. Borussia Dortmund are in danger of learning that the hard way this season.

Captain Mats Hummels' campaign started well enough, with vintage performances against Borussia Monchengladbach and Bayer Leverkusen making Dortmund supporters believe their No. 15 had left his nightmarish form in 2014/15 behind. His displays have, however, gradually deteriorated to a point where the question about whether he should still be considered an automatic starter for his side isn't hyperbolic.

Head coach Thomas Tuchel has to balance Hummels' undeniable qualities and his ever-so-apparent faults.

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Hummels' fall from grace since the 2014 FIFA World Cup is astounding.

"In Brazil he was at his best, showcasing to the world his skills as a defensive leader who also contributed in attack," noted Jonathan Harding for Deutsche Welle. Hummels rose to the pinnacle of fame through the tournament. Linked to seemingly every big club in Europe thereafter, the elegant centre-back looked certain to take the next step and become a global star and establish himself as one of if not the very best defender in the world.

Not even 18 months later, few would argue Hummels is far from being the best defender at his own club.

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His performances in the club's two defeats in 23 games across all competitions this season left a lot to be desired. In the 5-1 loss against Bayern Munich, he and centre-back partner Sven Bender were too slow to react to long balls from Jerome Boateng twice, allowing Thomas Muller and Robert Lewandowski to score.

On this past matchday at Hamburger SV, Hummels scored an own goal that effectively put the game to bed, prompting Bleacher Report's Rob Lancaster to list him as a loser of the game: "Mats Hummels was part of a Borussia Dortmund defence that looked to have more holes than a colander."

The Germany international even struggled in some wins. We highlighted his poor performance at FK Qabala in the UEFA Europa League, in which his leaving his position time and time again presented the underdogs with a number of goalscoring opportunities in the first half. In the recent Revierderby against local rivals Schalke 04, Hummels was easily the worst player on the pitch, gifting the Royal Blues their first goal while being partly at fault for the second as well.

More alarming than his mistakes per se is the fact Hummels is repeating errors.

After Dortmund conceded two goals off long balls at Bayern thanks in large part to Hummels' bad positioning, the 26-year-old made the same mistake on international duty with Germany just days after that game at the Allianz Arena. He allowed Republic of Ireland striker Shane Long to run by him and score the winner when a simple long ball from goalkeeper Darren Randolph flew over his head.

His tendency to leave his spot has been well documented, and Hamburg took advantage, as Harding argued: "His venturing out of defense (not as bad as in other games) left an already high back line exposed, and his sluggish to non-existing marking for Lewis Holtby's goal was verging on unacceptable for him."

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The only case the struggling skipper's staunch supporters can make is the fact Hummels is very important to Dortmund's buildup play. The alternatives, namely converted defensive midfielder Sven Bender and Neven Subotic, are similar to Hummels' regular partner, Sokratis Papastathopoulos: good, aggressive defenders, but not players to initiate attacks from the deep parts of the pitch.

In recent weeks, however, Hummels' quality in buildup play has seemingly regressed. He's often one of the first of the Black and Yellows to lose patience, overhitting almost every long ball—his specialty in better times—or taking players on with audacious, if not reckless, dribbles.

All things considered, a compelling argument can be made that Tuchel should indeed bench his captain. Hummels would be better off getting a break in order to refocus and to come back stronger.

In many ways, Dortmund can't afford to overlook his temporary form because of his permanent class.

Lars is a featured columnist writing on Borussia Dortmund. He also writes for Yellowwallpod.com. You can follow him on Twitter.