Manuel Lanzini has suffered a serious knee injury in training which appears likely to rule the playmaker out of the World Cup.
The 25-year-old helped West Ham climb the Premier League table in 2018 and earned a place in Argentina's World Cup squad.
But Lanzini now appears set to miss the tournament, which begins next week, after Argentina said he tore the anterior cruciate ligament in his right knee in training.
Argentina said on Twitter on Friday: "Manuel Lanzini suffered, in today's morning training, the rupture of the anterior cruciate ligament of his right knee."
Such an injury usually leads to an absence of months, rather than weeks.
Lanzini, who scored five goals in 29 appearances for the Hammers in 2017-18, has made three appearances for Argentina, scoring his first goal in March's 2-0 victory over Italy.
Argentina are in Group D at Russia 2018, playing Iceland on June