The Japan international recently voiced his desire to return to his former club in the future and BVB have stressed the door is always open for the midfielder
The attacking midfielder joined United from BVB in the summer of 2012, but struggled to live up to the high expectations in his first season at the Premier League champions.
Kagawa recently admitted that he intends to return to Dortmund at some point in the future and Watzke insists the 24-year-old would be welcome to return to Signal Iduna Park - though he ruled out an imminent transfer.
"The door's always open for Shinji. A transfer is not an issue at the moment, though," Watzke was quoted as saying by Bild.
Kagawa has a contract with United until the summer of 2016, but his agent Thomas Kroth has refused to rule out a premature exit.
"At the moment, I expect Shinji to stay at Manchester United. However, you never know what will happen in football..." he stated.
The midfielder netted six goals in 20 Premier League appearances for United last season, although a knee injury sustained in the autmun restricted his playing time during the campaign.