The ability for Major League Baseball to complete a 60-game schedule during a pandemic met a new challenge on Friday when the coronavirus forced one-fifth of the day’s games to be postponed just one week after opening day.

ESPN reported Friday night that commissioner Rob Manfred told Tony Clark, the executive director of the players’ union, that the league could shut down for the season if baseball does not do a better job managing the coronavirus. The shutdown, players told ESPN, could come as soon as Monday if there’s a spike this weekend in positive tests or if players do not follow the league’s safety protocols.

The Cardinals said two players tested positive on Thursday while the team was playing in Minnesota, forcing Major League Baseball to postpone their game Friday night in Milwaukee. The teams are scheduled to play Saturday night. The Marlins are unable to play the Nationals this weekend after 18 of their players and two coaches tested positive for COVID-19, and the Phillies’ series with the Blue Jays was postponed when two Phillies’ clubhouse staffers and one coach tested positive.

The Phillies have not played since Sunday and the team said activity at Citizens Bank Park remains suspended until further notice. No new positive tests were reported Friday from Thursday’s testing, but their season remains on an indefinite pause.

No Phillies players have yet tested positive and the team is continuing with daily testing to monitor the fallout from last weekend’s series against the Marlins.

The Phillies have played just three games and will have to play 57 games in 56 days beginning on Monday if they are to complete a 60-game season by Sept. 27. Playing 60 games remains an unlikely objective, but the Phillies will likely try to near that threshold by playing a sizable number of doubleheaders if their season is allowed to resume.

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Major League Baseball announced Friday that doubleheaders will consist of two seven-inning games “due to dynamic circumstances.”

“Both the clubs and the players have determined that this step promotes player health and safety,” the league said in a statement.

Major League Baseball said its postponement of Friday night’s Brewers-Cardinals game “is consistent with protocols to allow enough time for additional testing and contact tracing to be conducted.” Yet the league allowed the Phillies and Marlins to play Sunday afternoon at Citizens Bank Park just hours after the Marlins learned that four players had tested positive in three days.

Marlins players decided Sunday morning in a group text message to play the game after learning about the positive tests. The organization agreed that they had taken proper protocols, informed Major League Baseball, and the game was played. Commissioner Rob Manfred said the team did additional testing, contact tracing, symptom checks, and temperature checks before the league “decided to proceed with the game on Sunday.”

The Marlins have since had 14 additional players test positive for COVID-19 and stayed isolated in Philadelphia until Friday, when the infected individuals returned to Miami via bus. Neither the Phillies nor the Marlins have played since Sunday and the pauses to their season have shuffled the schedules of four other teams. The Yankees, Orioles, Nationals, and Blue Jays were forced to postpone games this week against either the Phillies or Marlins.

The Cardinals’ last game was Wednesday night against the Twins at Minnesota’s Target Field. The Twins hosted the Indians on Thursday and Cleveland used the same visiting clubhouse that the Cardinals occupied. The Twins and Indians met Friday after learning that two Cardinals players tested positive in Minnesota, but decided to play Friday night.

While six teams were kept off the field Friday, the 24 other major-league teams planned to play as the league continues to operate with two teams paused indefinitely and two others postponed for at least one night.

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A week ago, the Phillies and Marlins celebrated opening day at Citizens Bank Park. A week later, they have played just three games while the rest of the league rolls on. And they are no longer the only teams forced to watch from the sidelines as baseball’s challenge deepens to complete a season.