Baltimore Ravens quarterback Lamar Jackson tested positive for COVID-19 on Thursday, sources confirmed to ESPN.

The reigning NFL MVP is the biggest star player to become infected in what has become one of the largest outbreaks in the NFL.

Jackson is among the four latest Ravens players and one staff member to test positive on Thursday, a source told ESPN's Adam Schefter. The Ravens now have had at least a dozen players test positive for the coronavirus this week.

This latest round of positive tests could threaten Sunday's game between the Ravens and the undefeated Pittsburgh Steelers, which already was postponed from Thursday night. Ravens coach John Harbaugh told players that they would not be allowed back to the team facility until Monday at the earliest in the interest of safety, a source said.

As of Friday morning, there has been no change in the status of the game and the league continues to monitor the situation, a source told ESPN's Jeremy Fowler. The NFL is trying to determine whether playing the game would be medically safe after five straight days of positive tests in Baltimore, and whether that could subside over the weekend.

Baltimore is down to two healthy quarterbacks: backup Robert Griffin III and practice squad player Tyler Huntley. No. 3 quarterback Trace McSorley is currently on the reserve/COVID-19 list.

With Jackson quarantining for 10 days, Griffin would start the next two games, if Sunday's game in Pittsburgh is played. Baltimore then faces the Dallas Cowboys on Thursday night. Griffin, the No. 2 overall pick in the 2012 draft, has made only one start in the past four seasons.

"Praying for my brother [Jackson] and every player, staff member and their families dealing with COVID-19," Griffin said in a tweet Thursday. "Ensuring the safety of the entire organization is important. Handling this outbreak within the team is bigger than football."

On Friday, teammate Calais Campbell tweeted: "We just want to contain this outbreak! Speaking from experience...you don't want to catch covid! This virus is brutal! I pray no one else has to go thru this. This is bigger than football."

According to a source, the Ravens believe Jackson was infected Sunday at M&T Bank Stadium, where the Ravens lost to the Tennessee Titans 30-24 in overtime, and tested positive only on Thursday. Jackson took snaps from center Patrick Mekari, who tested positive on Wednesday. Jackson's game-day locker is close to those of running backs J.K. Dobbins and Mark Ingram II, both of whom tested positive Sunday night.

Losing Jackson delivers another blow to the NFL's top-ranked rushing attack. Jackson along with Dobbins and Ingram -- who are already on the reserve/COVID-19 list -- represent 74% of Baltimore's rushing total this season (1,187 of 1,605).

Jackson will officially go on the reserve/COVID-19 list on Friday. This marks the fifth straight day that Baltimore has had positive tests.

The Ravens already have eight players on the reserve/COVID-19 list: Ingram, Dobbins, Mekari, defensive ends Campbell and Jihad Ward, nose tackle Brandon Williams, outside linebacker Pernell McPhee and offensive lineman Matt Skura.

This has been a tumultuous season for Jackson and the Ravens, onetime preseason Super Bowl favorites who have fallen to 6-4 after losing three of their past four games.

A year after becoming the league's second unanimous NFL MVP, Jackson struggled to match the same success, especially in the passing game. His 20.5-point decline in Total QBR from last season (an NFL-best 83.0) to this year (62.5) is the largest in the NFL. He ranks 22nd in passing yards (1,948) and 28th in completion rate (63.4%).

Jackson has increasingly expressed his frustration this month, from saying defenders have been calling out the offensive plays to declaring the Titans "wanted [the win] more than us" last Sunday. His supporting cast hasn't been as strong either. The offensive line has constantly changed because of injuries, and the wide receivers have already dropped as many passes this season (six) as all of last year.

The loss of Jackson means five season-opening starters on the Baltimore offense will either be on injured reserve (left tackle Ronnie Stanley and tight end Nick Boyle) or the reserve/COVID-19 list (Jackson, Skura and Ingram).

News of Jackson's positive test was first reported by NFL Network.