Last month, Los Blancos announced the signing of Diomande as they lured the RB Leipzig winger away from Liverpool in a deal worth around £120M.
The 19-year-old winger fully announced himself last season, registering 12 goals and nine assists in 33 Bundesliga matches.
However, he has only just been handed his first start under Mourinho as he played 72 minutes against Rayo Vallecano on Saturday afternoon.
Madrid won 4-1 but Diomande failed to grab a goal contribution, frustrating many supporters who are waiting for the Ivory Coast international to shine in front of goal.
Despite the criticism, Mourinho has defended the teenager who turns 20 in November as tensions begin to rise.
"Diomande is improving progressively. He hasn't been involved in top-level European football very long. He's had the last season at Leipzig and not much more.
“He has a lot to learn from a tactical point of view. But you can see he has incredible potential in one-on-ones, his crossing, and his shooting.
“He's a very strong player who gives us a lot of options. He can play in Vinícius Junior's position, or on the right ... I want to see Diomande evolve, because I know what the other players can give me," he explained.”
He later said: “Diomande is working hard to improve and has great potential.”
Diomande is competing with Vinicius Junior, Brahim Diaz and Arda Guler for a place on either flank under Mourinho who remains patient for now as the side pick up another win.