The league said that no other team has been in contact with the FC Dallas delegation since its arrival.
All of the league’s 26 teams are sequestered in hotels in advance of the tournament played without fans at the ESPN Wide World of Sports complex at Disney World. The tournament opens Wednesday.
Atlanta United plays against the New York Red Bulls on July 11 in Orlando.
MLS shut down because of the coronavirus pandemic on March 12, after the league's teams had played two games.
The league said on Sunday that 18 players and six staff members had tested positive for the virus before teams departed for the tournament.
League protocol requires players and staff to be tested every two days upon arrival in Florida. Teams began traveling last week in advance of the tournament.
FC Dallas is scheduled to open the tournament with a group stage match against the Vancouver Whitecaps on July 9.
» More AJC coverage of the coronavirus outbreak