Sources told NFL Network, the Baltimore Sun and ESPN on Thursday that Jackson was the latest of at least a dozen Ravens players and staff members to test positive for the coronavirus over the last week. Four Ravens players and a staff member tested positive on Thursday.
Players were told Thursday that the team's practice facility will remain closed for at least five days. NFL COVID-19 protocol requires players who test positive to self-quarantine for 10 days, which puts Jackson's status in doubt for the Cowboys game.
The Ravens issued discipline earlier this week to a strength and conditioning coach for not reporting symptoms and not consistently wearing a mask or tracking device, in connection to the team's outbreak.
Baltimore was scheduled to face Pittsburgh on Thanksgiving, before the game was postponed until Sunday to give the NFL more days to test players and attempt to contain the spread.
Veteran quarterback Robert Griffin III is expected to start in place of Jackson on Sunday in Pittsburgh. Ravens running backs Mark Ingram and J.K. Dobbins are among the other key offensive players out for Sunday's game due to positive COVID-19 tests.
The Ravens have placed 10 players on the reserve/COVID-19 list. Jackson will be officially added to the list on Friday.
"Praying for my brother [Jackson] and every player, staff member and their families dealing with COVID-19," Griffin tweeted Thursday.
"Ensuring the safety of the entire organization is important. Handling this outbreak within the team is bigger than football."
Jackson -- the reigning NFL MVP -- has completed 63.4% of his passes this season for 1,948 yards, 15 touchdowns and six interceptions. He also has 575 rushing yards and three rushing touchdowns in 10 games.
Griffin has completed just 1 of 2 passes for nine yards and has an interception in three appearances this season. The 2012 Offensive Rookie of the Year and Pro Bowl selection completed 60.5% of his throws for 225 yards, one score and two interceptions in seven games last season.
The Steelers host the Ravens at 1:15 p.m. EST on Sunday on NBC.