Watzke announced at the end of July that 'the door is always open' for Kagawa at the Bundesliga outfit.
The Japan international spent two years with Dortmund before moving to Old Trafford in the summer of 2012.
It had been suggested that he could be heading back to Germany after just 12 months in England, but Kagawa has played down those reports and committed himself to United.
Dortmund accept that they will have to be patient with any efforts to bring the 24-year-old back onto their books, but Watzke believes such a switch will be made at some point.
He told Westdeutsche Allgemeine Zeitung: "It's a fact that Shinji said on his own accord that he can imagine a transfer back to our club in the future. That is delightful news for our club.
"Shinji Kagawa is a great player, he had great success at our club, and we would definitely consider him if this ever becomes a possibility."