The Bundesliga champions were made to work for their 2-0 victory over the basement club at the Eintracht-Stadion.
It took 75 minutes for Bayern to break the deadlock through Claudio Pizarro before Mario Mandzukic made sure of the points four minutes from time.
The result ended Bayern's three-game winless streak in the league, which satisfied the German international midfielder.
"Today was a real test of our fighting spirit," Muller said.
"We coped well with the challenge, but it was a very tough game and the atmosphere was fantastic. We didn't play that well, but the opposition made it very difficult for us.
"We fought well, though, and remained patient."
Pep Guardiola's men had tasted back-to-back defeats to Borussia Dortmund and Augsburg, an uncharacteristic run of results for the German giants.
Prior to that, Bayern surrendered a two-goal lead as they drew 3-3 with high-flying Hoffenheim and club captain Philipp Lahm was delighted to finally get back on the winners list.
"It feels good to be travelling back home with the three points in the bag," he said.
"I thought we played well, particularly in the second half. In the end, we did well to get the victory."
Bayern must now switch their attention to Wednesday's UEFA Champions League semi-final first leg against Real Madrid at the Bernabeu.
And goal-scoring hero Pizarro believes Saturday's result will give Bayern a much-needed boost heading to Madrid.
"It was important for us to win before the game against Real Madrid - it gives us confidence," he said.
