Brazilian player Valdivia was on Saturday substituted at half-time after testing positive for coronavirus.
The midfielder was tested by his team Avai ahead of their match against CSA, with a view to confirming the player's availiblity for their following game, against Juventude on Tuesday.
However, Brazilian Football Confederation (CBF) informed the club that a positive test was returned during Saturday's Brazilian Serie B game at CSA and the player was substituted at half-time.
A club statement from Avai read: "At half-time the CBF communicated... that Valdivia tested positive," Avai said in a statement adding that the player was asymptomatic.
"That is why Valdivia was substituted. The club is now reviewing his isolation and return."
— Avaí Futebol Clube (@AvaiFC) January 16, 2021
Head coach Claudinei Oliveira added: "We wanted to do the new test [on Saturday] evening or Sunday, but the laboratory said that it would only be able to deliver the results if the tests were taken early on Saturday.
“To our surprise, the result came during the half-time break. A person from the laboratory went to great lengths to deliver it.”
Oliveira went on to suggest foul play in light of the speed of the result.
He continued: "We will have to understand what happened to Valdivia. We cannot judge anyone's character, but I have never seen a result come out so fast.
"It seems that the laboratory is owned by the vice-president of [opponents] CSA. They committed to deliver the results on Monday and did it in record time, perhaps, to harm us in some way."
Valdivia was replaced by Renato, who went on to score the equaliser for Avai in the 1-1 draw.
Brazil reported 61,567 cases of the new coronavirus on Saturday, and 1,050 deaths, the fifth consecutive day with more than 1,000 fatalities, the Health Ministry said.
More than 200,000 people have died of Covid-19 in Brazil.
Additional reporting by Reuters.