The Germany international hopes BVB can return to their usual best again in the years to come and has admitted that Louis van Gaal was vital to his career
BVB finished 25 points behind Bayern in the Bundesliga table last term, while they are trailing Pep Guardiola's men by 23 points this year after previously winning back-to-back titles in 2011 and 2012.
"Dortmund are no longer on par with Bayern, but are now team that can make life difficult for us. At some point, Dortmund were the team that spurred us on to improve," Muller was quoted as saying by Abendzeitung.
"It is important that the situation changes again. I don't think things will always be like this, where we could be crowned Bundesliga champions as early as March.
"Things will get tighter at the top again."
Muller then went on to discuss his special relationship with Louis van Gaal and admitted that the Dutchman has played a big role in his career.
"Van Gaal is a special kind of guy, a special person. He has been very important to my career. We had a special relationship during his time at Bayern."
Van Gaal coached Bayern between 2009 and 2011.