The ninth-year player suffered the injury toward the end of Cincinnati's practice at Welcome Stadium in Dayton, after making contact with cornerback Dre Kirkpatrick.
First-year coach Zac Taylor was hesitant to give any indication of the severity of the injury.
"We'll find out a little bit more about him later on," Taylor said. "I don't want to speculate on anything. We'll find out more."
Green's left shoe was removed before he departed the field. Last season, he missed seven games with a right toe injury.
That campaign marked only the second time Green did not have at least 1,000 receiving yards in a season during his career. He participated in Bengals minicamp in June.
Green, 30, is entering the final year of his contract.
"I felt like he had a good summer," Taylor said. "Sometimes, today, freak things happen."
The Bengals were playing in unfamiliar territory Saturday as the team opened training camp in Dayton in conjunction with the NFL's commemoration of its 100th season. Triangle Park in Dayton was the site of the first NFL game, between the Dayton Triangles and Columbus Panhandles in October 1920.
The remainder of the Bengals' training camp practices will be in Cincinnati.