The official Bundesliga Twitter account posted the following on Wednesday morning:
It's terrible news for both club and country, but Jurgen Klopp will undoubtedly be the one feeling the effects of his player's absence more so, considering he has already lost centre-back Neven Subotic to injury.
The Black and Yellows boss now heads into this weekend's encounter against Bayern Munich without either of his first-choice central defenders.
Berlin-based journalist Ryland James said that it was a torn ankle ligament keeping Hummels from first-team action, although UEFA.com relates the problem to his heel. January is earmarked for his likely return date at this point:
Hummels has been an omnipresent force for both his club and national team in recent years, but has suffered similar ankle issues in the past, according to Transfermarkt.co.uk.
That being said, the 24-year-old has so far succeeded in overcoming his injury obstacles and will look to bounce back from this latest setback in similar fashion.
Klopp's side are currently four points behind Bayern in the German top flight and have Bayer Leverkusen stalking them in third place, level on points with the 2010/11, 2011/12 champions.
The slightest slip-up could mean an opening for Sami Hyypia's Factory Squad, who would be more than happy to pounce on the opportunity.
Dortmund now have to dip into their reserves and will be looking toward the likes of Sokratis Papastathopoulos and others to fill in during Hummels' and Subotic's absences.
Including the Bayern clash, Dortmund have five more league matches before breaking up for the winter period just before Christmas, resuming play in late January.