Dortmund star Sancho continues to be linked with a big-money move to Premier League giants United, who have reportedly identified the England international as a top potential addition.
The Red Devils tried to lure Sancho to Old Trafford during the off-season, however, they were unsuccessful in bringing the 19-year-old back to England after he left Manchester City for Dortmund in 2017.
Asked about Sancho's future beyond 2020, Zorc told Kicker : "No decision has been made. But you don't have to be a prophet to know he won't play here for another five years."
Sancho has established himself as one of Europe's most promising talents after leaving Premier League champions City two years ago.
He scored 12 goals and supplied 14 assists as Dortmund finished runners-up to Bayern Munich in the Bundesliga last season, while Sancho already has struck three times and set up another four in the top flight this term.
"He has taken this positive development [at Dortmund]. I doubt that would have been possible with top English clubs," Zorc said. "We trust the players very early, and we are not afraid to let them play. He acknowledges that."
Zorc added: "He's an instinctive footballer, a street footballer, he's always a footballer, but he's always setting new limits. He does not sit back satisfied. He wants to play football successfully and beautifully. That alone is his motivation."
Dortmund are eighth in the Bundesliga after six games, three points adrift of leaders Bayern following back-to-back 2-2 draws against Eintracht Frankfurt and Werder Bremen in which they dropped points from winning positions.
Following a creditable 0-0 draw against Barcelona in their first game - in which Marc-Andre ter Stegen saved Marco Reus' penalty - Dortmund take on Slavia Prague next.
However, Sancho and his team-mates would do well not to underestimate a side that have conceded just two league goals this season and held Inter to a 1-1 draw, a game they were minutes away from winning before Nicolo Barella snatched a stoppage-time equaliser.