The Borussia Dortmund centre-back is convinced los Blancos will provide his side with the sternest of tests in Europe on Wednesday evening
Having gone 18 games unbeaten in La Liga, Madrid's season has been turned on its head in recent weeks with back-to-back defeats to Barcelona and Sevilla seeing them drop to third place in the table.
Nevertheless, despite Real's indifferent form, Hummels is aware of the task facing a resurgent Dortmund side at the Bernabeu.
"I watched their game against Barcelona. You cannot talk about a crisis after that match," the Germany international told the Bundesliga's official website.
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"Real Madrid were by no means outplayed and they have a lot of world-class players. But they're still beatable if you know their weaknesses."
Hummels then went on to stress that BVB cannot afford to solely focus on Cristiano Ronaldo as Madrid have much more to offer up front.
"There's more to Real Madrid than just Ronaldo.
"They've got Karim Benzema, Angel di Maria and Gareth Bale too. That's a lot more than just one player."
Wednesday's first leg takes place at the Santiago Bernabeu, with the return at the Signal Iduna Park scheduled to take place on April 8.