Fabian Johnson left Borussia Monchengladbach's Europa League game with an apparent injury on Thursday, less than a week before he is expected to report to the U.S. national team for a pair of key World Cup qualifiiers.
Johnson came off in the 16th minute against Schalke and was replaced by Jonas Hofmann.
Gladbach coach Dieter Hecking said after the game that Johnson had suffered a thigh injury, while Germany's Christoph Kramer had a suspected knee injury.
The 29-year-old was named to Bruce Arena's U.S. squad to face Honduras and Panama over the international break that begins next week.
His loss would be a major setback for the Americans, who are already without injured right-back DeAndre Yedlin and Eric Lichaj.
Arena has previously said that Johnson is "best suited to be in the midfield," and has contemplated playing three men on the back line. The top options at right-back are Michael Orozco and Geoff Cameron.
Gladbach crashed out of the Europa League after Johnson left Thursday's game, despite taking a 2-0 lead. Schalke rallied after half-time to secure a 2-2 draw and advance on away goals.