Frank Augstein/Associated Press
The fashion in which Juergen Klopp's men conceded Firmino's opener was a poor display of their structure, with Hoffenheim's high line taking full advantage of their overlap in numbers and scoring all too easily.
However, the most culpable part of Dortmund's line-up for these mistakes sat in the engine room, where Milos Jojic and Nuri Sahin were doing their best at holding a strong, anchoring presence.
For the first half of the season, defence was by far Klopp's biggest concern when it came to player absences, but with Sven Bender, Sebastian Kehl and of course Ilkay Gundogan still on the injury table, things are most dim at the team's core.
The duo could muster a passing percent average of just 78 percent between them, according to Squawka (Sahin at 76 percent and Jojic at 80 percent), and the hosts' grip on possession may have been all the stronger were more capable assets in place.
Though tipped for bigger things in their respective futures, Sahin and Jojic simply weren't up to scratch on this occasion, bringing to light a substantial weak point that the club will look to solve prior to the DFB-Pokal final against Bayern Munich in a fortnight's time.