Centre-Back Is Borussia Dortmund's Biggest Area of Concern Ahead of New Season

Borussia Dortmund stand at the dawn of a season that could well be defined by transition. After a sometimes hectic summer saw them sell five and sign no fewer than eight players to this point, there's no telling how long the team will take to jell and become a cohesive unit.

Pre-season performances were naturally up and down—thanks to prolonged vacations following Euro 2016 and the Olympic football tournament, the Black and Yellows have yet to train with the entire team—and, as this writer detailed in an earlier piece, it wouldn't be too surprising if that continued early in the campaign.

The impact of the defections of three key players can't be overstated, as the Ruhr side have suffered losses in all areas on the pitch.

The incredible production of Henrikh Mkhitaryan, who was involved in a whopping 55 goals across competitions last year, per Transfermarkt, will be hard to replace, but early indications are Dortmund have done well in replacing the Armenian with talented youngsters Ousmane Dembele and Emre Mor, as well as club-record signing Andre Schurrle.

In midfield, head coach Thomas Tuchel will be without mastermind Ilkay Gundogan. It was no stretch to call the Germany international the most important player in the 42-year-old's system last season—when healthy. Seeing as he missed a number of games towards the end of the season, including all but eight of the 90 fateful minutes at Anfield in the Europa League quarter-final and the DFB-Pokal final against Bayern Munich, his loss arguably weighs a little less.

Dortmund haven't signed a like-for-like replacement, with Sebastian Rode coming in as a dynamic box-to-box midfielder and returnee Mario Gotze potentially taking up some of Gundogan's playmaking responsibilities.

Arguably the club's biggest loss, however, was the transfer of Mats Hummels back to boyhood club Bayern. The 27-year-old leaves a void few players in the world—and even fewer in Dortmund's price range—could have filled.

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A world-class centre-back oozing with creativity on the ball, the 2014 FIFA World Cup winner was also the club captain for the last two years, a natural progression for a player who was a leader from a young age at Dortmund. He also served as a face of the club, especially since Jurgen Klopp left and the less charismatic, somewhat unapproachable Tuchel took over in 2015.

Having sold the Germany international for a whopping €35 million fee, per Transfermarkt (link in German), the Black and Yellows have so far only reinvested a fraction of that money in a replacement, with Marc Bartra coming in for €8 million (in German).

The 25-year-old has left a positive first impression in pre-season, with confident and reliable performances in defence while showing the technical qualities fans will have expected from a player coming over from the Blaugrana.

However, time will tell how the Catalan will fare in filling Hummels' gigantic shoes. After all, there's a reason he was available for what feels like a bargain: He played in so few matches "that a stipulation in his contract saw his release clause reduced from €40 million," as Juan Jimenez noted for Spanish sports daily AS.

Bleacher Report's Karl Matchett noted Bartra's lack of playing time last season: "Indeed, the Spaniard played the fewest minutes out of the five centre-backs available to Luis Enrique in 2015-16, most of which came in largely irrelevant fixtures where wins came easily, or as a late sub to boost his appearance tally."

Having fallen behind the unimpressive duo of Thomas Vermaelen and Jeremy Mathieu in Barca's centre-back depth chart, the 25-year-old will have to make a big jump to replace Hummels.

Dortmund have to hope the transition period for the Spain international will go smoothly, as there's little proven depth behind him. 

Bartra looks set to start the season next to veteran defender Sokratis Papastathopoulos, coming off an up-and-down campaign that ended in a spectacular performance in the DFB-Pokal final, even though he missed a penalty in the shootout. 

The 28-year-old at times lost his spot to converted defensive midfielder Sven Bender, who showed better synergy with Hummels last season. With Bender in Rio de Janeiro on international duty, however, and the Greece international showing good form in pre-season, Sokratis should be relatively safe at right centre-back.

A pairing of the man they call Papa and Bender, however, doesn't seem like a good idea. Dortmund would lack a ball-playing defender who can take on playmaking responsibilities. That leaves Matthias Ginter and newly signed Mikel Merino as back-up options for Bartra.

Twenty-year-old Merino has struggled with the transition from midfielder for Osasuna in Segunda Division to centre-back under Tuchel in pre-season and doesn't seem like a viable option in the immediate future. He lacks defensive instincts, often rushing out of his position rather needlessly, exposing the space behind him for runners.

It's easy to see why Dortmund think of him as a centre-back for the future, given his strength in the air and overall physicality, as well as his good passing range from deep positions on the field, but he looks like a long-term project at this stage.

Ginter, finally, is also on Germany duty at the Olympics at the moment and remains at the centre of near constant transfer speculation. Dirk Krampe of local newspaper Ruhr Nachrichten (link in German) reports that VfL Wolfsburg are steadfast in their pursuit of the 22-year-old despite sporting director Michael Zorc's refusal to put the versatile defender on the market.

Earlier in the summer, Patrick Berger of Hamburg-based tabloid Morgenpost (link in German) wrote that Ginter had already held talks with officials of Hamburger SV. It's the second summer in a row that the World Cup winner has flirted heavily with leaving Dortmund, after he was close to joining Borussia Monchengladbach last season, per Krampe.

It's only understandable that the 22-year-old thinks about leaving, seeing as he's unlikely to tie down a regular starting spot with the Black and Yellows, he's rather a back-up for multiple positions. Even if he stays, as seems likely at the moment, his Dortmund career doesn't suggest he can fill Hummels' role on the field.

Per Transfermarkt, only 17 of his 60 matches for the Black and Yellows have come at centre-back, and most of those came under Klopp—Tuchel used him at the heart of defence in only six matches last campaign.

Somewhat curiously, Dortmund seem content with their personnel at centre-back, after the signing of Omer Toprak from Bayer Leverkusen failed to materialise this summer.

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Chief executive Hans-Joachim Watzke hinted at a pre-agreement with the former Turkey international for next year in an interview with Reviersport, as the 27-year-old reportedly can leave the Rhein side for €12 million due to a release clause, per Kicker (links in German).

Toprak himself was keen to move, telling local newspaper Kolner Stadtanzeiger (h/t Stephan Uersfeld of ESPN FC): "I have told the club what I think. There was a bid from Dortmund, which was rejected by Leverkusen."

It's quite baffling that Dortmund balked at a reported €3 million difference (via Spox.com, in German) between their offer and Bayer's demands for a player of Toprak's quality, especially considering they later splashed the cash on Schurrle, who almost seems redundant given the club's strength on the attacking wings.

With that, the Black and Yellows left themselves vulnerable at a crucial position. Not only do they have to hope Bartra can confirm encouraging pre-season performances in important games after playing sparingly over the last few seasons, they also have to hope their senior options can stay healthy.

It's a risky strategy that will make the club's decision-makers look like geniuses if the cut-price Catalan impresses—and like fools if Dortmund's defence proves to be leaky in the coming months.

Lars Pollmann also writes for YellowWallPod.com. You can follow him on Twitter.