Real Madrid's miserable run of results continued as they slid to a shock 1-0 La Liga defeat at Alaves and failed to score in a fourth-straight game.
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Barcelona began the new La Liga campaign with a 3-0 win over Alaves on Saturday with the help of a spectacular under-the-wall free kick from Messi.
The Ghana international signed a four-year deal with the Spanish side on Monday.
Morocco's football federation has lost its case against FIFA to recruit Spain forward Munir El Haddadi before the World Cup.
Atletico Madrid stayed mathematically in the La Liga title race, for a few hours at least, with a 1-0 victory over Alaves.
Alvaro Medran, on loan from Valencia, showed his frustration over his lack of playing time at Alaves by crying during a postgame interview.
Morocco's hopes of adding Munir El Haddadi to their World Cup squad have been dashed by FIFA.