Midfielder Brenden Aaronson was removed from FC Salzburg's starting lineup 10 minutes before its Austrian Bundesliga championship match at Wolfsburg on Sunday, calling into question whether he will be available for the United States' upcoming World Cup qualifiers.
"Last-minute change to our starting XI," Salzburg said. "Brenden Aaronson had to pull out due to knee problems, unfortunately."
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A source told ESPN that there is no structural damage and only stiffness and soreness on Sunday. There is optimism that Aaronson will still be able to play during the upcoming window, though the next couple days will be telling.
The 21-year-old from Medford, New Jersey, is the only U.S. player to appear in all qualifiers, starting seven. He scored against Canada and Honduras in September.
Aaronson has three goals in 21 league matches this season.
Seeking to return to the World Cup after missing the 2018 tournament, the U.S. is at Mexico on Thursday, hosts Panama three days later in Orlando, Florida, and closes at Costa Rica on March 30. Training starts Monday in Houston.
Canada leads North and Central America and the Caribbean with 25 points, while the U.S. is second with 21, ahead of Mexico on goal difference. Panama is fourth with 17 points, followed by Costa Rica with 16 and El Salvador with nine.
ESPN's Kyle Bonagura contributed to this report.