Portuguese top-flight club Vitoria Guimaraes confirmed that three players have tested positive for coronavirus.
Teams in Portugal's Primeira Liga have resumed individual training to prepare for the resumption of the season, scheduled for the end of May.
The players are "asymptomatic and quarantined," the club said in a statement on Saturday night, without naming them.
The cases were detected during a screening on Friday of all the club's staff, coaches and players.
The Portuguese government announced on 30 April that the Portuguese first division would be allowed to resume from the last weekend in May, behind closed doors and subject to the approval of a health protocol.
Sports and health authorities are still working on this protocol and have not yet confirmed the date for the resumption of matches, with ten rounds still to be played.
When the competition was interrupted by the coronavirus pandemic, Porto were at the top of the table, one point ahead of Benfica.
"It is important that the sports seasons can end," Prime Minister Antonio Costa said on Friday during an interview with Porto Canal.
Players and coaches "will be the first to take the necessary precautions to avoid being contaminated", he added, stressing that "if a club has all its team infected, it will forfeit the match".
As well as the three cases reported by Guimaraes, at least one other player had already tested positive last week, from the under-23s of the Lisbon club Belenenses.