3 Biggest Worries for Borussia Dortmund's 2015/16 Season so Far

Borussia Dortmund have enjoyed a very positive start to the Thomas Tuchel era. Under their new head coach, they've won 12 of their 16 matches in all competitions. They sit second in the Bundesliga table and lead their Europa League group with two games against the weakest opposition on paper, Gabala FK, on the horizon.

But not all is sunshine and roses for the club.

A spell of four games without a win, including their first loss under Tuchel, against Bayern Munich, showcased some of Borussia's shortcomings. Some of them were evident in Friday's win at Tuchel's former club, Mainz 05, as well.

With that in mind, here are the three biggest worries for Dortmund's 2015/16 season so far.

Defensive Vulnerability

Dortmund have kept a clean sheet in seven of their 16 games this season. While that stat sounds pretty solid, three of those came against the likes of Wolfsberger AC and Chemnitzer FC in Europa League qualifiers and the DFB-Pokal. 

Excluding those three games against lowly competition, Dortmund have conceded 18 goals in 13 matches.

Matthias Schrader/Associated Press

Dortmund's playing style under Tuchel should not be conducive to a large number of goals conceded. They see the majority of the ball in most games and concede almost the same number of shots in the Bundesliga as Bayern, 7.2 per game, according to WhoScored.com.

But Bayern have only conceded four goals in the Bundesliga. The staggering difference lies in the percentage of shots on target conceded by the two sides. For Dortmund, almost every second shot they concede finds its way on target; for Bayern, that number is closer to 33 percent (statistic accumulated from WhoScored.com).

Dortmund have to find a way to get that number down, as it has already hurt them in the league, as ESPN FC's Michael Caley noted after the 2-2 draw with newly promoted Darmstadt 98.

"

xG map for Dortmund-Darmstadt. Yup, that's 2 goals from 3 shots for Darmstadt. pic.twitter.com/uTikMQ6zL5

— Michael Caley (@MC_of_A) September 27, 2015"

Lack of a 2nd Consistent Goalscorer

Pierre-Emerick Aubameyang's fine early-season form has overshadowed another of Dortmund's problems: They have only one truly consistent goalscorer in their ranks.

In the Bundesliga, Aubameyang's 10 goals and three assists mean he has been involved in 54 percent of his side's goals.

PATRIK STOLLARZ/Getty Images

Dortmund's reliance on Aubameyang's goalscoring could come back to haunt them should the Gabon international ever hit a dry spell. During the recent run of three winless games in the Bundesliga, Aubameyang was the only Dortmund player to hit the net, securing his team draws against Hoffenheim and Darmstadt. 

If Dortmund are to have a successful campaign, others will have to step up to the plate at some point. 

In a first step in that direction, Dortmund's next-best goalscorers in the league, Henrikh Mkhitaryan with four and Marco Reus with three goals, both scored against Mainz 05 on Friday. The Armenian hadn't scored since the middle of September and Reus, who was injured in the meantime, since late August.

Heavy Reliance on Key Players

Aubameyang isn't the only player Dortmund rely too heavily on. The aforementioned Mkhitaryan and Julian Weigl are just as irreplaceable as the Gabonese because they are the "main structure-supporting agents," as Tom Payne wrote for Spielverlagerung.com.

Dortmund's second string hasn't shown much so far, which is why Tuchel has been reluctant to rotate his team. While the squad is strong and deep, as every position looks filled with a quality backup on paper, some players cannot be replaced without there being a significant drop in performance. 

Michael Probst/Associated Press

If something were to happen to Weigl, for example, Dortmund would be in serious trouble. With Nuri Sahin still injured, Dortmund don't have another defensive midfielder combining Weigl's vision and passing ability with defensive strength available. 

Ilkay Gundogan is another example. His return to world-class form has given Dortmund a massive lift. Gonzalo Castro, signed as his replacement when it seemed like Gundogan would leave the club in the summer, has been rather disappointing so far. Gundogan's intelligence, vision, dribbling and passing abilities would be dearly missed.

Unless Tuchel finds a way to get some of the second-stringers playing at a much higher level, Dortmund need to hope their key players stay injury free. Losing just one of them could have a devastating effect on the club's season. 

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