The young winger scored eight goals and registered the same number of assists in all competitions last season and popped up on the radar of several big clubs.
Leverkusen sold Kai Havertz in the summer for a fee of around €80m and could have raked in more money for Diaby if they wanted to sell him.
According to German magazine Sport Bild, Manchester United expressed an interested in taking the winger to Old Trafford.
The Premier League giants indicated that they would be prepared to offer €50m for the player in order to get their hands on him.
But Leverkusen refused to even enter into negotiations with Manchester United after his agent made the club aware about the interest from England.
Leverkusen were in no mood to sell him and wanted to prevent themselves losing more top players after letting Havertz move to Chelsea.
Diaby has a contract until 2024 with Leverkusen and for the moment the club do not want to sell him.
It remains to be seen if Manchester United will test Leverkusen’s resolve in the rapidly approaching January transfer window.
