Green performed Saturday night at the Michigan International Speedway after NASCAR's Carfax 250 race. He's best known for the 2003 hit "Wave on Wave."
The Jackson Citizen Patriot reports that late in his show, he shouted, "Anyone got a beer?"
A can of suds suddenly soared from the audience onto the stage. Then a second can from the crowd hit Green between the eyes, knocking him out.
Medical personnel treated him at the track.
A phone message seeking an update on Green's condition was left Tuesday at his publicist's office in Nashville, Tenn.
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