The 22-year-old attacking midfielder exited with an apparent leg injury in the 25th minute of a 4-0 friendly win against Nicaragua at Asuncion, Paraguay.

Paraguay head coach Gustavo Alfaro said Enciso will undergo tests and he remained "hopeful" that the injury was not muscular.

"Julio suffered two simultaneous injuries; a tight hamstring from impact and then on his waist, which affected his quadriceps area," Alfaro said. "He got scared and that's why he came off."

Enciso, who scored 12 goals this season for Strasbourg in France's Ligue 1, has four goals in 31 career caps heading into Paraguay's first FIFA World Cup since 2010.

"When he took the first hit, he signaled to me and I told him to come off. He replied that it was loosening up, but then he took another blow to the waist and that's when he felt the injury," Alfaro said.

"We hope Julio is alright and he can be ready to return for the opener."

Paraguay faces United States in its Group D opener next weekend at SoFi Stadium in California.

Paraguay also will take on Turkey on 20 June (AEST) and Australia on 26 June (AEST), both at Levi's Stadium in California.