Atlanta United defender Miles Robinson was stretchered off the field after collapsing to the artificial turf with a non-contact injury to his left leg on Saturday in Atlanta's 4-1 win over the Chicago Fire.
"I think it's going to be a bad injury," Atlanta coach Gonzalo Pineda said at halftime.
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A serious injury to the 25-year-old central defender could cost him a roster spot at the World Cup, where the U.S. opens on Nov. 21 against Ukraine, Scotland or Wales. Robinson started 10 of the Americans' 14 World Cup qualifiers.
He took a step back near the midfield stripe to head a ball that had been headed by Chinonso Offor in the 14th minute. With Stanislav Ivanov closing in but not making contact, Robinson headed the ball down and kicked it forward with his right foot as he landed awkwardly and fell.
Robinson pointed at his left leg with his both arms, pounded the turf and pointed at his leg again as athletic trainers came out to attend to him.
He was removed on a stretcher in the 16th minute.
Robinson has three goals in 21 international appearances. He scored in the 1-0 win over Mexico in the CONCACAF Gold Cup final last August.
Atlanta already has lost goalkeeper Brad Guzan (right Achilles), Osvaldo Alonso (right ACL) and Dylan Castanheira (left Achilles) to season-ending injuries. Star forward Josef Martinez had arthroscopic surgery on his right knee that the team said on April 8 would sideline him for six-to-eight weeks.