Rojo, who was never shown a red card in his seven seasons in Manchester, tried to plead his case with referee Fernando Rapallini, but in truth gave the official little choice with a late, rash challenge when already on a booking.
River went on to win the match 2-1 thanks to first-half goals from Julian Alvarez, before Carlos Zambrano scored a consolation goal for Boca in stoppage time.
The result returns River to the top of the table after 14 matches, with Boca down in ninth place.
Fans in Argentina were allowed back into stadiums over the weekend for the first time in 18 months as Covid-19 restrictions were eased further.
“People were detained for the sale of false tickets … and the Public Ministry of Buenos Aires city is going to investigate whether the capacity was exceeded,” a spokesperson for the office in charge of major events told Reuters.
Argentine media estimated the attendance was somewhere between 52,000 and 57,600, well over half of the 72,000 capacity of the Monummental, River’s home ground.