Vinicius Junior cannot recover his form. The Real Madrid star is enduring a thoroughly modest season.
Jose Mourinho keeps handing the Brazilian winger chances, yet Vinicius remains far below the level expected from a player of his abilities and standing.
Sport newspaper report that Vinicius once again disappeared from view against AEK Athens, just as Diomande keeps growing in confidence.
He started against AEK Athens and played the full 90 minutes, but failed to make a difference. His suspension in La Liga is nearing its end, and his two appearances in the Champions League do not inspire optimism.
The decline in his level coincides with the technical explosion of Yan Diomande. The Ivorian is going through a wonderful spell on the attacking front, and he also puts in a great effort supporting his team-mates defensively. Those two things specifically are the areas for which the Brazilian faces the greatest criticism.
In Greece, Diomande confirmed that his best position is the left wing, the very spot occupied by Vinicius. That could push the Brazilian onto the bench sooner than expected.
Mourinho has corrected many things. He restored Jude Bellingham to his level, moved Fede Valverde to right-back and worked on developing Diomande. Vinicius, though, remains the outstanding task facing the Portuguese, and not for a lack of confidence.
The Brazilian has sat on the bench in only one match all season, starting almost every game for which he was available.
Caution has shaped Mourinho's approach. He has chosen to protect the Brazilian, directing no criticism at him and placing no responsibility on him, whether in press conferences or on the pitch.
The manager keeps firmly in mind the dispute that arose between Vinicius and Xabi Alonso, and how that tension ended with his predecessor's departure.
So he has handled the situation with wisdom and flexibility. He has substituted him in many matches, yet always kept his focus on winning the player's cooperation and preserving their relationship.
Last Wednesday in Athens offered the best example. With Vinicius one of Real Madrid's worst players on the night, Mourinho took off Diomande and kept the Brazilian on the pitch, waiting for a reaction.
Fans and journalists called for Vinicius to come off. The Portuguese stuck to his decision and kept giving him the chance. He knows he needs the best version of Vinicius if the team are to compete for every title.
Even so, the credit of confidence in the Brazilian has begun to run out within the corridors of the Santiago Bernabeu. More and more people no longer see him as an untouchable starter once he returns from suspension, and that could take the problem to another level.
With Diomande in the side, the team gain balance, because the Ivorian provides significant support to Marc Cucurella. With Vinicius, the Spanish full-back carries additional burdens. It showed clearly in Athens, as it has in other matches this season.
Mourinho now has an extremely sensitive situation to manage. The Brazilian knows he is going through a bad spell, but right now he sits behind Diomande in the pecking order. Dropping him could spark a fresh flashpoint of tension.
The decision is anything but easy. Benching one of the team's most famous and influential players carries a cost, inside the dressing room and in the club's surroundings, and Mourinho knows a precedent for it all too well.
Keep him in the starting line-up on the basis of his standing and stardom, though, and you penalise the player who is most ready right now, while disrupting the balance the manager has worked so hard to build.
Vinicius has proven in previous seasons that he has the tools to climb out of similar slumps. The window available to him, however, is narrowing.
His return to La Liga is a great chance to restore his image and remind everyone why he was once among the best dribblers and difference-makers in the world. Fail to seize it, and Diomande is waiting for his turn. Mourinho, however keen to protect Vinicius and give him confidence, will find fewer reasons to keep waiting.