Having gone 18 games unbeaten in La Liga, Real's season has been turned on its head in recent weeks after back-to-back defeats to Barcelona and Sevilla.
Carlo Ancelotti's men have dropped down to third in the table, behind Madrid rivals Atletico and Barcelona, though they improved to thrash Rayo Vallecano 5-0 on Saturday.
Despite Real's indifferent form, Hummels is aware of the task facing a resurgent Dortmund side, who are three games unbeaten, at the Bernabeu.
"I watched their game against Barcelona and I don't think you could call that bad," the Germany international told the Bundesliga's official website.
"Real were by no means outplayed and they have a lot of world-class players, but they're still beatable if you know where their weaknesses are."
Dortmund know exactly where Real's weaknesses are after defeating the Spanish giants 4-3 on aggregate en route to last season's final at Wembley.
World Player of the Year Cristiano Ronaldo has enjoyed another productive season in Madrid, netting 28 goals in La Liga and a competition-high 13 in the Champions League.
But Hummels is wary of Real's full array of other attacking threats, including Karim Benzema and Angel di Maria, who have registered five assists apiece in this season's European campaign.
"There's more to Real Madrid than just Ronaldo," Hummels added.
"They've got (Karim) Benzema, (Angel) di Maria and (Gareth) Bale too. That's a lot more than just one player."
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