Grujic is on a season-long loan at Hertha Berlina this season.
He told Kicker: "He is still very interested in the Bundesliga. He wants to know everything.
"Jurgen is still a big Dortmund fan. When I was there, Hertha had just drawn 2-2 at BVB. He did not like that and was upset!"
Grujic has returned from an ankle injury and is eager to return to action.
"It has not been easy for me to see the boys playing in the last few weeks. It's a nice feeling to be back. I waited six weeks for that."