Dortmund parted company with Tuchel at the end of last season, with Dembele since demonstrating his quality when he scored for France against England during their 3-2 friendly win at the start of this month.
Tuchel signed Dembele from Rennes last summer, with the 20-year-old scoring 10 goals and providing 21 assists in all competitions for Dortmund.
But the France forward is now said to be seeking a move, though a move to Barcelona is not on the horizon, despite some reports to the contrary.
Eurosport reports: “According to our sources, he has been badly affected by the exit of Tuchel, the man who brought him to Germany from Rennes for €15m on a five-year contract a year ago.”
Their story credits Man Utd and Chelsea with an interest, despite Dortmund being likely to demand £70million for the former Ligue 1 Young Player of the Year.
Dortmund are already facing the prospect of losing Pierre-Emerick Aubameyang, with the Bundesliga club having named their price for the striker.