The 31-year-old left-back was attacked Sunday as he left the Beira-Rio stadium in the southern city of Porto Alegre, Xinhua quoted Radio Gaucho as saying.
No further details were given and it is not known if Santos was hurt.
“This citizen has to be arrested,” Flamengo coach Ney Franco said. “I hope that the police at the stadium are taking the appropriate measures.”
The loss left Flamengo last in Brazil’s Serie A standings with seven points from 11 matches.
Santos joined the Rio de Janeiro club from Arsenal last year after a brief loan spell at Gremio.