Christian Benitez died of heart failure, according to his club side Al Jaish.
The Ecuador international died on Monday shortly after being admitted to hospital with severe stomach pain, hours after making his debut for the Qatari club.
A statement from Al Jaish read: "We have received the medical reports, which were issued by official state entities and stated that the sudden death was caused by heart failure."
Ecuador Football Federation president Luis Chiriboga said on Tuesday that the 27-year-old had been taken to the hospital with sharp stomach pains, developed peritonitis and died of cardiorespiratory arrest.
Benitez, nicknamed Chucho, joined Al Jaish earlier last month from Mexico's Club America, after previous spells with El Nacional, Santos Laguna and Birmingham.
Al Jaish also announced Benitez's body will be returned to his home country once legal issues are cleared.
The statement continued: "The club will bear all expenses of transferring the body from Doha by private aircraft.
"We promise the farewell and transfer of the body will be at highest level that meets the late player's name and reputation.
"All players and club affiliates will participate in the ceremony. The club management again assures its endless support to Christian's family and stand by their side during this hard time."