Sancho is thought to be near the top of Uniteds wish list this summer, with the Red Devils linked with a deal worth over £100m to sign the England international.
However, speaking to Bild, Watzke stated that Uniteds interest in Sancho is not a problem as the winger will definitely remain with Dortmund next season.
He said: “It can be assumed that Jadon will not play more than ten years at BVB. But we have clear agreements about the present.
“Jadon is not on the market and plays next season guaranteed at Borussia Dortmund.”
After Uniteds disappointing season, they are thought to be on the lookout for exciting young talents who can help them bounce back. The likes of West Hams Declan Rice and Chelseas Callum Hudson-Odoi have been linked with the club, but Sancho is said to be their top priority.
The 19-year-old racked up 13 goals and 19 assists in all competitions for Lucien Favres men, as they came incredibly close to beating rivals Bayern Munich to the Bundesliga title.
This kind of form, which earned Sancho a place in Gareth Southgates England squad, has seen the former Manchester City youngster emerge as one of Europes most exciting young talents. Alongside United, City, Real Madrid and Barcelona have also been linked with Sancho, who would likely cost in excess of £100m.
The Red Devils are expected to spend heavily this summer, meaning paying such a fee for Sancho would likely not be a problem, especially if a number of first-team players are sold to help raise funds.
However, after failing to qualify for next seasons Champions League, many felt as though winning the race for Sanchos signature would be incredibly difficult, and now Watzkes latest admission suggests United may have to look elsewhere.