After testing positive for what was initially reported as a “social drug”, Peru’s football captain, Paolo Guerrero is maintaining his innocence. A source close to the player has told ESPN Brazil that Guerrero was completely surprised by the positive test results, and that Peruvian captain is insisting that the test result was trigged by flu medicine.

The initial positive test came after Peru’s World Cup qualifier against Argentina on October 5th. According to reports, four players underwent mandatory doping tests following the match in Buenos Aires: Argentina’s Paulo Dybala and Alejandro Gomez, and Guerrero and Edison Flores of Peru.

Peru’s soccer federation, the FPF, confirmed the initial suspension on Twitter late on Friday afternoon, saying that while they supported FIFA’s decision to suspended Guerrero, they stand by their captain. They did not disclose which substance he tested positive for, but did say that they believe follow-up tests will clear Guerrero.

On Friday, Fernando Solera–who acts as the head of doping control of the Brazilian Football Confederation, and as a doping official for FIFA–said Guerrero could still provide evidence that he did not take prohibited substances without warning Peru and Flamengo beforehand.

“If Guerrero has justification for this and that justification was offered before the match against Argentina, it is not a doping case,” Solera told channel SporTV. Solera also mentioned that Guerrero’s club, Flamengo, showed evidence that the forward did not take any illegal substances during his time with the Brazilian club. Guerrero had also been ill in lead up to the following qualifier, against Colombia on October 8th, which could help his case in proving that the test picked up the medicine given to him before the qualifiers.

Guerrero’s provisional 30-day suspension will keep him out of the country’s intercontinental World Cup playoff matches against New Zealand over the next two weeks. In his place, Peru has since called up Yordy Reyna, from MLS’ Vancouver Whitecaps. The first-leg of the qualifier playoffs against New Zealand will be played at Westpac Stadium on Saturday, November 11, while the second leg will be played at the Estadio Nacional de Lima in Peru’s capital on the 15th.

If Guerrero does pass the follow-up drug tests–or if he is able to provide evidence that the result was due to flu medicine–he will be able to play in the World Cup should Peru advance. If not, he could be facing a ban ranging from 6 months to up to 4 years, effectively ending the 33-year-old striker’s career.