However, Mainz coach Sandro Schwarz insists Diallo, 21, will not be leaving before next week's transfer deadline.
"I'm really zero-thinking about this subject, I am not afraid that Abdou will leave us again!" he declared. "He's smart enough to know the next step for his development - the decision is made.
"He will not leave out of impatience!"
Mainz snapped up Diallo just last summer from AS Monaco for €5m.