United were heavily linked with the England international towards the end of last season, but Sancho remains a Dortmund player ahead of the Bundesliga kick-off this weekend.
The former Manchester City winger enjoyed a superb season at Signal Iduna Park in 2018/19, scoring 12 goals and providing 14 assists in Germany's top flight.
And Watzke has admitted that an elite club - thought to be United - enquired about his services a few months ago, only to be told that the youngster was not for sale.
"There aren't many 19-year-olds with such a potential," Watzke told Ruhr Nachrichten. "He is also not a player from the region or one who would have any connection to it.
"When you have a player like Jadon Sancho, you must reassess the situation every single year. Everything else would not be honest. If a foreign player is not convinced that the club is right for him at the exact time, it just does not make any sense.
"A chief of one of the superclubs asked me back in spring if there was a chance [to sign Sancho] but I told him straight away he should forget about it and he never contacted me again. He knew I meant what I said."
Dortmund begin the 2019/20 campaign against Augsburg on Saturday.
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