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

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

Jackson is among the four latest Ravens players and one staff member to test positive 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 was already 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.

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

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