Sané, 24, has impressed under the tutelage of Pep Guardiola at Man City, winning the PFA Young Player of the Year award in 2017–18, but Heynckes has cast doubts over whether he is worth the mooted €100m fee.
“For me, he has not yet made a breakthrough to being an absolute top player,” Heynckes told German outlet Die Welt.
“I think this is a player at a crossroads. Will he keep working hard on himself, or will he remain just as an exceptional talent?
“He has to be ready to make the next big step. Because there is no use in flashing the talent from time to time. I also honestly tell you that I don’t consider the price justifiable.”
The treble-winner’s comments are unlikely to deter Bayern, who have made the Germany flyer a top summer target and replacement for Franck Ribéry.
The veteran Frenchman ended his long stay at the Allianz Arena last August when he joined Fiorentina in Italy.