Reyna, who had only just returned from a hamstring injury that kept him out six months, was replaced by Julian Brandt as Dortmund went on to steamroll Gladbach by a final score of 6-0.
The 19-year-old injured his right hamstring in the World Cup qualifying match between the U.S. and El Salvador on Sept. 2, and had played three times for Dortmund across all competitions since returning, including Sunday's start.
"Gio is really upset, you can see it in the pictures," Dortmund coach Marco Rose said. "He's just back from injury and was really in good shape. I was, we were all happy that he was back. It was important for us to have him on the field with the quality he has."
Rose said the club will do all it can to get Reyna back to his best.
"We'll get the boy back and we'll give him every support because we need him, because he's an outstanding athlete, a great person," Rose said.
Reyna came on as a substitute in Dortmund's 5-2 loss at home to Bayer Leverkusen on Feb. 6, but missed last weekend's win at Union Berlin due to illness before also coming on a sub in a loss to Rangers in the Europa League last Thursday.
Reyna looked sharp against Gladbach on what was just his fifth league appearance of the season, testing goalkeeper Yann Sommer with a good effort early on. He sustained the injury around the time Dortmund captain Marco Reus opened the scoring in the 26th.
The U.S. is set to begin its final three matches of CONCACAF World Cup qualifying on March 24 against rivals Mexico, with the Americans in second place on 21 points and ahead of third-placed El Tri on goal difference.
Reyna has not featured for the U.S. since the Sept. 2 injury, and his availability for the March qualifiers on the road to Qatar will now be in doubt following Sunday's setback.
Information from the Associated Press was used in this story.