The Germany international has urged BVB to keep things tight at the back and believes anything is possible for the Bundesliga side
The Schwarzgelben recorded a 2-1 home win over Madrid in the group stages and Hummels has insisted that they cannot afford to concede a goal again in Wednesday's match in Dortmund.
"It would be a good result if we win and keep a clean sheet. Because then every goal we score in Madrid would work to our advantage," Hummels told the official Dortmund website.
"We need to get a good result if we don´t want to find ourselves under pressure from the start in the second leg.
"We know how difficult it will be to come out winners from these two games. But we also know that everything is possible for our team. And we know that we need a good result to have a realistic chance."
Hummels then went on to stress that Dortmund will have to focus on stopping more than just Cristiano Ronaldo as Madrid have much more to offer, but remains confident about BVB's chances.
"Real are a lot more than Cristiano Ronaldo and have a top player in every position," he said.
"But that partly applies to us as well. It will be crucial that we attack and defend as a whole team. If all 11 players have the same idea we are very difficult to beat."
Wednesday's first leg at the Signal Iduna Park is scheduled to kick off at 20:45CET.