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Depth has proven critical to the success of many big teams in recent years, especially as schedules have become increasingly congested to accommodate extra international matches and tournaments like the Club World Cup.
Dortmund are most certainly lacking in depth, and their fatigue following Champions League matches has certainly played a role in their standing 17 points behind Bayern in the Bundesliga table. In theory, BVB should be at a disadvantage in the Champions League as well. However, they in reality are coping quite well.
Mats Hummels missed Tuesday's match due to influenza, and Felipe Santana filled in with an impressive performance, nodding in the opening goal. This was not the first time that BVB needed to call on reserves: Ilkay Gundogan was injured for the first leg, which ended in a rather favorable result, and in the group stage a B-team beat Manchester City at the Signal-Iduna Park.
While injuries are unavoidable, suspensions are another issue. But with eight matches having been played, no BVB player is in any danger of a ban. Only six Dortmund players have been booked in the Champions League this season (a tournament low), and no player has received more than one caution.
Dortmund have an incredible disciplinary record given the quality of opposition they've faced, but it speaks well of the club's team defense and ability to control games without having to resort to rash challenges. And it means they have one less reason to worry about their limited depth.