Dortmund were unbeaten in their first seven league fixtures under Peter Bosz and sat five points ahead of Bayern at the summit, but a drastic change in fortunes has seen them triumph just once in their last six in all competitions.
Their slump has coincided with the Bavarians launching a winning run since Jupp Heynckes replaced Carlo Ancelotti as coach, Saturday's 3-1 Klassiker victory at BVB their seventh in a row.
Bayern now lead the way in the Bundesliga after 11 matches, sitting four points clear of RB Leipzig and six ahead of Dortmund, which Robben can scarcely believe.
"We are back on track but nothing more than that. We have to keep our feet on the ground," said Robben, who opened the scoring with his 93rd Bundesliga goal.
"We can be very happy because since the new manager came in, we were five points behind Dortmund and now we are six points ahead. If you told me that a few weeks ago I would have said you were crazy.
"We know what [Heynckes] can do. The players have to do it on the pitch but he gives structure, tells us how to play and is very clear about things and I think we are more stable now."
Christian Pulisic was one of few bright points for Dortmund, the teenage forward playing a key role in Marc Bartra's consolation strike two minutes from time by keeping the ball alive in the corner.
He was disappointed with how Bosz's side started the game but tried to remain positive about their chances of getting back on track after the international break.
"We didn't come out ready to play in the first half. Bayern were doing whatever they wanted, finding the spaces," said Pulisic.
"Second half we had more intensity but it just wasn't enough. We didn't come out ready and against a good team like them it's not possible to win like that.
"We've got to keep our heads up, keep fighting, go to training and try to make it right. We need the support from our own guys, from our staff and from our fans – it's very important.
"These runs don't last forever so things will get better."