United States striker Folarin Balogun scored for the third game in a row then left the pitch injured as Monaco seized command of the French league by winning at Rennes 2-1 on Saturday.
Balogun netted the winner in the 22nd minute but was forced off after the hour mark with an apparent shoulder injury. It was unclear whether he would be able to join Mauricio Pochettino's U.S. squad ahead of matches against Panama and Mexico.
Unbeaten Monaco suffered throughout the second half but kept its small advantage intact to move to the top of the table. Lagging three points behind, Paris Saint-Germain can close the gap in Nice on Sunday.
Thilo Kehrer put the visitors in front in the sixth minute and Ludovic Blas leveled for Rennes just five minutes later.
Balogun was picked out by Eliesse Ben Seghir with a low pass on the edge of the box. He quickly turned and accelerated to lose his marker before finishing with a fine left-footed chip over goalkeeper Steve Mandanda.
Blas' goal was impressive too; a 25-meter strike that left Monaco goalkeeper Philipp Köhn no chance.
New USMNT head coach Pochettino has Ricardo Pepi, Josh Sargent and Haji Wright as striker options in his squad, if Balogun is unable to recover in time.
The U.S. will face Panama at Q2 Stadium in Austin, Texas, on Oct. 12, with a match against bitter rival Mexico at the Estadio Akron in Guadalajara three days later.
Information from The Associated Press contributed to this report.
