Baltimore Ravens star quarterback Lamar Jackson tested positive for the coronavirus, part of the team’s ongoing outbreak that is jeopardizing this weekend’s already postponed game in Pittsburgh and could lead to the NFL’s regular season being extended by a week.

The positive test result for Jackson, the NFL’s reigning MVP, was confirmed by a person familiar with the situation. According to another person with knowledge of the matter, the Ravens had four additional players test positive for the coronavirus in all.

The Ravens-Steelers game remained scheduled, as of Thursday night, for Sunday at 1:15 p.m. Eastern time, according to a person familiar with the NFL’s planning. But the additional positive test results for the Ravens returned Thursday night further imperiled that game, given the NFL’s stance that it must be confident that transmission of the virus is contained before a game is played.

The game already has been pushed back once after originally being scheduled for Thanksgiving night.

“Praying for my brother [Jackson] and every player, staff member and their families dealing with COVID-19,” Ravens backup quarterback Robert Griffin III wrote on Twitter. “Ensuring the safety of the entire organization is important. Handling this outbreak within the team is bigger than football.”

If the Ravens-Steelers game is further delayed until Monday or Tuesday, that would affect the Ravens’ scheduled game Thursday night against the Dallas Cowboys. If the Ravens-Steelers game cannot be played in Week 12, the NFL could be forced to add a Week 18 to the regular season. League leaders have said they would do that if all games cannot be played in the current 17-week framework. The Ravens and Steelers already have had their bye weeks, so the game could not be rescheduled for later in the season without the addition of a Week 18.

Whenever the Ravens play next, they are likely to be without Jackson. Under the NFL’s treatment protocols, a player with a confirmed coronavirus case is placed in isolation and cannot rejoin team activities for at least 10 days. The timeline for the player’s return depends on further testing results and the development of any symptoms.

Griffin, the former NFL offensive rookie of the year for Washington, would be in line to start in Jackson’s place. With third-string quarterback Trace McSorley already on the team’s covid-19 reserve list, practice squad quarterback Tyler Huntley could be Griffin’s backup.

Earlier Thursday, a person with knowledge of the testing results said one Ravens player and “a couple” of staff members had tested positive in the results returned from Wednesday’s testing. The Ravens reportedly continued to work remotely Thursday. The team did not respond to an earlier request for comment on the specifics of the testing results.

Approximately a dozen Ravens players and at least seven staff members have tested positive in recent days.

“NFL will have a hard time playing through this one,” former Green Bay Packers executive Andrew Brandt wrote Thursday night on Twitter.

Allen Sills, the NFL’s chief medical officer, told league-owned NFL Network on Wednesday that the league had “done a very, very deep dive into this situation” through interviews, review of the data and genomic sequencing, and it believed the end of the outbreak was nearing.

“We feel like we have a really good handle on exactly when transmission occurred and how it occurred,” Sills said. “And I think that we feel like we’re just a couple days away from being out of that window of vulnerability for that transmission event. So obviously you have to take each day as it comes and look for any new data that may emerge. But right now we simply feel like that as of [Thursday] we would not have confidence in going forward but I think we feel we’re very close to the end of that transmission event and at present we’re targeting Sunday, absent any new information that would change the facts that we have right now.”

The Ravens announced Wednesday evening that they were disciplining a staff member for actions related to the team’s coronavirus cases. The team declined to provide details. The NFL is leaving open the possibility of disciplining the Ravens, according to a person close to the situation. The league previously fined the Tennessee Titans $350,000 and fined the Las Vegas Raiders $500,000 and stripped them of a sixth-round draft pick for protocol violations.

There has been no indication to this point that the NFL would consider a forfeit in this case. But NFL Commissioner Roger Goodell warned teams earlier that protocol violations resulting in adjustments to the schedule could result in game forfeits or the loss of draft picks.

The Atlanta Falcons and Cleveland Browns announced they were operating remotely Thursday because of positive tests. The Falcons said they had positives by two staff members not on the coaching staff. The Browns said they had a positive test by a player. The Falcons are scheduled to host the Raiders on Sunday; the Browns are slated to play at Jacksonville.

The Arizona Cardinals placed wide receiver Larry Fitzgerald on their covid-19 reserve list Thursday. That list is for players who test positive and those determined through contact tracing to have been exposed to the virus. NFL Network reported that Fitzgerald tested positive. The Cardinals play Sunday at New England.