MLS Multiplex was the first to report the extent of Long's injury.
Long, 28, was injured deep into second half stoppage time of the Red Bulls' 1-0 loss to the Philadelphia Union on Saturday, as he landed following an aerial challenge. He immediately crumpled to the turf and grabbed his right ankle. After being attended to by a trainer, he was unable to put any weight on his right leg as he was helped off the field.
The injury comes at a particularly inopportune time for Long. He has long had the trust of U.S. manager Gregg Berhalter, with the player's pace at the back especially valued. The U.S. also has a busy slate of national team games coming up, starting next month with the CONCACAF Nations League finals, as well as the beginning of World Cup qualifying in September. Among the candidates to replace Long are Anderlecht's Matt Miazga, Hoffenheim loanee Chris Richards, Nashville SC's Walker Zimmerman and Atlanta United's Miles Robinson.
Long has also long been angling for a transfer to a European club, while the Red Bulls were hoping that the defender could help ease the transition to manager Gerhard Stuber, who is in his first full season with the club.
Long has been a mainstay for the Red Bulls since breaking through during the 2017 season, making 140 league and cup appearances, scoring nine goals. At the international level, Long has made 21 appearances for the U.S., scoring three goals.