Video footage published by newspaper Folha de S. Paulo shows three metallic plates falling into the pitch just minutes before the match between local rivals Cruzeiro and Minas Futebol, reports Xinhua.
Nobody was hurt in the incident.
According to the meteorological office, the weather conditions were not abnormal for Brazil's southeastern city of Belo Horizonte at this time of year.
A met office spokesman said 10.6 mm of rain was experienced over three hours while winds did not exceed 36 km/h.
Cruzeiro manager Marcelo Oliveira also criticised the state of the water-logged pitch.
"Questions have to be asked. Is this pitch up to FIFA standard? I think the quality needs to be better," he said.
The Mineiro stadium, which has been redeveloped at a cost of $300 million, will host six matches during this year's World Cup.
Saturday's incident has renewed concerns about the safety of Brazil's 12 World Cup venues.
Seven stadium construction workers have died so far while an October fire caused structural damage at Cuiaba's Arena's Pantanal, according to local prosecutors.