According to Marca, Vinicius's absence left Mourinho hunting for the right position for the Ivorian, and the answer came on the right wing. Diomande had already flashed his quality there against Rayo Vallecano and Elche, standing out in both games.
The real breakthrough arrived against Villarreal. Mourinho left Vinicius on the bench, and though the Brazilian came on in the second half, he ended up being sent off. Diomande, meanwhile, turned into a constant menace down the left and capped it with a wonderful goal.
He has not budged from that position across the four matches since. A goal and an assist arrived against Sevilla, then another goal against Racing.
That same spell saw him play on the left against Leipzig, where he scored again and set up a decisive pass. Vinicius's return pushed him back to the right in the last outing against AEK Athens, and he left no mark on the scoresheet before coming off in the 67th minute.
Wearing the number 10 for his country, Diomande played the full match in Ivory Coast's attack, mainly on the left wing alongside Elie Wahi and Nicolas Pepe. From that flank he won the ball that created one of his nation's two goals against Gambia.
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LaLiga Matches
Athletic Bilbao vs Espanyol
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Rayo Vallecano vs Valencia
Sevilla vs Real Betis
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Barcelona vs Villarreal
Racing Santander vs Real Sociedad
Osasuna vs Malaga
Getafe vs Atletico Madrid
Levante vs Elche
The moment of truth for Mourinho
Vinicius's four-match suspension in La Liga handed Diomande the chance to stake a claim to the Brazilian's spot.
Now Vinicius is back for the next match against Celta Vigo at the Santiago Bernabeu on Saturday 21 November.
Marca spelled it out in their report: "The time has come for Mourinho to make a decision, as he has always been accustomed to relying on Vinicius on the left flank and Diomande on the right flank. It is natural for the situation to continue this way if both players feature together in the starting line-up."
The dilemma runs deeper, though. Fielding both alongside Kylian Mbappe means sacrificing someone in a more withdrawn role, and Jude Bellingham, Arda Guler and Bernardo Silva have all stepped up in recent weeks.
The Leipzig model
Mourinho is convinced Diomande produces his best on the right wing. This is a player who leans heavily on his right foot.
At Leipzig last season, he racked up far more minutes on the right, 1,591 in total, against only around 478 on the left.
Play him in that natural position and he hands Mourinho real attacking depth. His dribbling, his pace and his strength let him burst through defences, reach the byline and whip crosses into the box.
On the left, by contrast, he mixes dribbling with cutting inside diagonally, carving out the space to load up his favoured foot and shoot.
Diomande has indeed scored for Real Madrid from that position, whether off a worked move or after receiving a ball from the right.
Mourinho touched on all this recently, careful not to brush aside the rest of his forwards, even as he backed Vinicius as the natural fit on the left.
The coach said: "Diomande is capable of playing on both sides, and so is Brahim Diaz, while Endrick leans more towards playing on the right side, but he can also play on the left. Among all of these, Vinicius is the most adapted to playing on the left wing."
Even so, the teamsheet will tell the real story of whether Vinicius keeps his place. The coach trusts him, yet just 10 days ago he refused to confirm in advance that the player would start.
Clearly the Portuguese had begun, even before the suspension, to hint at his concerns over Vinicius's dip in form. The Brazilian now knows he faces a fight to win his place back.
