Mintzlaff, who chairs Leipzig's supervisory board, delivered an unambiguous public statement after the club's 3-1 Bundesliga win over Union Berlin, in which Diomande was named man of the match.
"I'm on the supervisory board and am not the sporting director. But I can say this: if I were the sporting director, I wouldn't sell this young player, who hasn't even been with us for a full season yet, no matter what price is offered.
"I believe he's a player who can still develop further, because he's still very young. And he can certainly become more expensive. The best argument is a long-term contract. Therefore, I'm completely relaxed about the whole matter."
Diomande has 20 goal contributions — 12 goals and eight assists — in 30 Bundesliga appearances since joining from Leganés for €20 million last summer. He is contracted until 2030.
Leipzig are working on inserting a release clause of between €70 and €80 million into a new deal, with a view to retaining him until at least 2027 and driving up any future fee.
However, the signals around the player point increasingly towards departure. Diomande recently switched agents, joining Roc Nation — the agency that also represents Mohamed Salah — in a move widely interpreted as preparation for a high-profile transfer. He has previously stated publicly that he wants to play for Liverpool and that his father's dream is to see him at Anfield.
Leipzig's asking price is understood to be in excess of €100 million. Whether any club will meet that valuation remains to be seen, but Mintzlaff's comments were a clear shot across the bows of those who believe the deal is close.