Julian Alvarez missed Atletico Madrid's training session on Wednesday, two days after the Rojiblancos players returned from a rest period.
Atletico opened their La Liga campaign with a 2-0 win over Malaga before drawing 2-2 with Villarreal at the Metropolitano.
An official Atletico statement, published by Spanish newspaper "Marca", said the Argentine striker was absent through illness.
Alvarez had come on as a substitute in the 66th minute against Villarreal, and the club's own fans jeered him.
His absence, the newspaper added, only sharpens the row over his future. That will be settled on 1 September, the moment the transfer window closes and his destination for the season becomes clear.
The forward had already announced his desire to leave Atletico during Argentina's run at the 2026 World Cup, a move that angered the Madrid club.
Barcelona lead the race for his signature, with the Catalans desperate to land him. Atletico, though, want to keep him and refuse to sell to the Blaugrana.
Diego Simeone's side now turn their attention to Saturday's third La Liga fixture against Sevilla.
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