The Cardinals said they learned about the positive tests late Thursday night and have instructed all players and staff to self-isolate in their hotel rooms in Milwaukee until further notice. The team also said nobody left for Miller Park on Friday morning.
MLB's quick decision to postpone Friday's game, which was scheduled for 2:10 p.m. ET, comes after the outbreak within the Miami Marlins, who have had 20 positive tests -- 18 by players over the past eight days after another on Friday, sources told ESPN.
The postponed game has been rescheduled as part of a doubleheader on Sunday, with the first game to start at 2:10 p.m. ET. The league and the MLB Players Association announced Friday that doubleheaders will now be seven innings in length for each game.
Saturday's game remains scheduled for 7:10 p.m. ET.
MLB said Friday's postponement allows enough time for the Cardinals to do additional testing and conduct contact tracing. A source told ESPN's Jesse Rogers that if what is happening with the Cardinals had occurred before the Marlins' outbreak, Friday's game at Miller Park would still be played.
"We are supportive of Major League Baseball's decision to postpone today's game and look forward to playing our home opener as soon as conditions safely allow," Brewers president/general manager David Stearns said in a statement. "The health and safety of our players and employees are, and will continue to be, our top priorities."
The Cardinals played in Minnesota on Tuesday and Wednesday before a scheduled day off Thursday. The Twins hosted Cleveland on Thursday night, meaning the Indians likely used the same visiting clubhouse as St. Louis. Minnesota is scheduled to host Cleveland again Friday night.
This brings the number of teams being held out of action on Friday to six. The Brewers and Cardinals will join the Marlins, Philadelphia Phillies, Washington Nationals and Toronto Blue Jays as teams affected by the fallout from positive tests.
The Cardinals' positives represent the first positive tests the sport has seen outside of the East Coast since the regular season started last week.
The Associated Press contributed to this report.