Injuries have forced midfielder Maurice Edu and left back Corey Ashe off the US national team roster ahead of two friendlies this week ahead of World Cup 2014 qualifying matches in June.
The US Soccer Federation announced the setbacks on Monday, two days before the Americans play host to Belgium with another match on Sunday at Washington against Germany, the homeland team of US coach Jurgen Klinsmann.
Edu has a sports hernia that was diagnosed after he returned to English Premier League club Stoke City following a loan to Turkish side Buraspor.
Houston's Ashe suffered a groin strain in a 1-1 Major League Soccer draw Sunday against Kansas City. He had been called in only for the Belgium match.
Another US midfielder for Stoke City, Brek Shea, dropped out with a strained right calf. He will travel to Dallas for treatment and return to the squad when he is fit but no timetable was set for his possible return.
North American World Cup qualifying resumes for the US side on June 7 at Jamaica with the Americans returning home to Seattle on June 11 to entertain Panama and to Salt Lake City to face Honduras on June 18.
The Americans are third on the qualifying table after three of 10 matches, with only the top three clinching berths in Brazil next year. The fourth-place team will play a two-game series against New Zealand for a trip to Brazil.