Diaby has been on Newcastle’s radar since last year when Eddie Howe wanted to take the Frenchman to St. James’ Park.
But Bayer Leverkusen were adamant about not wanting to lose him and priced him out of a move to Newcastle.
The Magpies have been keeping an eye on him again with a view to potentially signing the winger, but there has been little progress in recent months.
And according to the German broadcaster Sky Deutschland, Manchester United have joined the race to potentially get their hands on Diaby.
Erik ten Hag wants to add more creativity and goals to his squad and signing a number 9 is a priority.
But the Manchester United manager is also prepared to bring in creative wingers and Diaby is a player the club are looking at.
He is being closely monitored by the Premier League giants but any deal is hinging on the club sorting out their ownership this summer.
Leverkusen want around €70m to €80m for a player who has scored 14 times in all competitions this season.