United is reportedly interested in signing England international Sancho and could make a move if Paul Pogba forces through a transfer to Real Madrid.
Ole Gunnar Solskjaer has already captured 21-year-old pair Aaron Wan-Bissaka and Daniel James this close season and, at 19, winger Sancho would fit the profile of a club looking to refresh a stale squad.
The teenager could command a starting XI spot at the Premier League club on the strength of his last season with Dortmund, during which he scored 12 goals in 34 Bundesliga appearances.
That level of exposure to first-team football has hastened Sancho's progress into the senior England set-up and leads former Red Devils forward Rooney to believe he is in the right place.
"Dortmund is a great club with great fans, they trust their young players and you can see that in Sancho and the progress he has made in the last 12 months," the DC United star said.
"It's fun to watch him, he's doing really well at BVB.
"Now Jadon has to ask himself whether he would get the playing time he gets in Dortmund when he moves to England, or would he just sit on the bench?
"Playing time is so important for a young player like Jadon, so if he gets the most playing time in Dortmund, he should stay there."
Sancho spent just over two years with United's rivals Manchester City before opting to move abroad.
He made his Bundesliga debut in October 2017 and broke into Gareth Southgate's England squad 12 months later.