Chelsea fullback Ian Maatsen is off the mark for Borussia Dortmund.
The Dutchman is on-loan at Dortmund to the end of the season.
Maatsen pounced on an error in the Union Berlin defence to score in the second-half and settle the game 2-0 in the visitors' favour.
The win sees BVB move into fourth place on the table.
Maatsen's loan doesn't include a permanent option.
