The Brazilian was subbed off in the first half of Sunday’s game with a bloodied patch visible on his shorts.
“The athlete Gustavo Henrique went through an evaluation with urologist Paulo Henrique, who recommended three days of rest and therapy with antibiotics,” said a statement from the club.
“He remains under care of the medical department.”
Henrique, a 27-year-old center-back who joined Flamengo from Santos earlier this year, also provided an update on his condition.
“Thank you very much to all. We’re together.”
Following the emphatic victory against Corinthians at the weekend, Flamengo is second in Brazil’s Serie A, level on points with first-placed Internacional.
The Rio-based side next faces Atletico Junior in the Copa Libertadores on Wednesday.