The season-opening series between the Washington Nationals and New York Mets was postponed Friday, with Nationals general manager Mike Rizzo saying a fourth player from the team has tested positive for COVID-19.

Along with the positive tests, five other Nationals players and a staff member are in quarantine from contact tracing, as the fallout from a COVID-19 outbreak has cast into question when Washington's season will begin. The Nationals are still awaiting the results of Thursday's tests.

"We're in crisis management mode," Rizzo said in a video call with reporters.

Positive tests from three Nationals players and a fourth player with an inconclusive result led to the postponement of their scheduled Opening Day game Thursday. Games on Saturday and Sunday were postponed as well and will be made up throughout the rest of the season, as the teams are scheduled to play 19 games against each other as National League East rivals.

When Washington plays first will depend on the results of contact tracing the team has undertaken in the wake of the outbreak. Nationals players and staff have remained isolated in recent days, and Rizzo said they will prepare to play their next series starting Monday against the Atlanta Braves.

The nine players who have tested positive or are quarantining would not be available if the Nationals play Monday and would be replaced with alternate-site players, Rizzo said.

Rizzo had said Thursday that one of the players testing positive had a fever, but the general manager said Friday that the player is feeling better and that no one else is showing COVID-19 symptoms.

"A virus is a virus. It's very contagious. So I think it could happen in many, many different ways. I don't think it's a breach of protocol or inadequate protocols," Rizzo said. "But it's something that happened. We're going to try to find out how it happened and investigate it and see if we can stop it from happening again."

The jointly negotiated protocol between Major League Baseball and the MLB Players Association mandates a weeklong quarantine for someone deemed to be in close contact with individuals who have tested positive, and those determined to have COVID-19 must isolate for at least 10 days.

Depending on the results of testing, the Nationals could work out Saturday, Rizzo said.

"You can't just go from a dead stop in this postponement to ramping up to play the Atlanta Braves and not expect [there] to be injuries," Rizzo said. "But these other players, it's just hard to have them sitting and playing video games or whatever for two or three days and then to ramp up again.

"To me, it's a safety issue and we've got to get these guys' blood flowing," he added. "We've got to get them moving around so they don't go into the season going from zero to 100 mph without preparation for the last couple of days."

The Mets will begin their season Monday in Philadelphia. The team plans to stay and work out in Washington the next two days before traveling to Philadelphia. The Mets won't make any changes to their rotation, with Jacob deGrom -- who had been scheduled to pitch Thursday -- set to start Monday's game.

MLB and the players' association released their weekly testing results Friday, saying three players and one staff member had tested positive for COVID-19 from 14,354 conducted tests.

Outbreaks on St. Louis and Miami last season disrupted the teams but ultimately didn't derail them, as both made the postseason. The Nationals, who have playoff aspirations, will start their season short-handed, as those who have tested positive will be placed on the COVID-19 injured list.

ESPN's Jesse Rogers and The Associated Press contributed to this report.