The league leaders succumbed to an Andrej Kramaric strike in the 21st minute of their visit to the Rhein-Neckar-Arena and could well have lost by a greater margin.
When asked whether the end of a seven-match winning streak had killed Bayern's momentum, Hummels told DFB.de: "Maybe, but it would have been different if we had lost it next week against Real Madrid. This is why I think this loss comes at a good time.
"It shows us that if we let our level drop even a little bit then we are not as strong as we have been in recent weeks. We got better, but it wasn't enough.
"Maybe we needed this wake-up call. It showed us that we are so much better when we are alert and actively pressing on the pitch."
With seven top-flight games left to play this season, Germany international Hummels urged his colleagues to remain focused on the task at hand.
"I don't believe that it is already over," he said. "As soon as it is mathematically over then it's over. Anything is possible in football.
"We have to make sure we accept the challenge from Dortmund on Saturday otherwise that cushion could be whittled down in the next few weeks."