BEREA, Ohio -- Cleveland Browns quarterback Case Keenum has tested positive for COVID-19, potentially leaving the team without its top two quarterbacks for Saturday's game against the Las Vegas Raiders, sources told ESPN's Adam Schefter and Kimberley Martin.
Starting quarterback Baker Mayfield already tested positive for the virus earlier this week and remains on the reserve/COVID-19 list.
Nick Mullens, who was just added to the active roster Thursday, is now in line to start at quarterback for the Browns. Mullens started 16 games for the San Francisco 49ers from 2018 to '20, but has yet to throw a pass for the Browns.
Keenum practiced for Cleveland on Thursday.
The NFL on Thursday relaxed the testing requirements to clear vaccinated personnel, potentially positioning the players on the list for a faster return. The league has said it has not discussed moving the Browns-Raiders game to another day.
Mayfield criticized the league on Thursday, tweeting: "@NFL Make up your damn mind on protocols. Showing up and making only 3 teams test?!? All so you can keep the game as scheduled to make money."
"Actually caring about player safety would mean delaying the game with this continuing at the rate it is .... But to say you won't test vaccinated players if they don't have symptoms, then to pull this randomly. Doesn't make any sense to me," he added in another tweet.
Actually caring about player safety would mean delaying the game with this continuing at the rate it is....
— Baker Mayfield (@bakermayfield) December 16, 2021
But to say you won't test vaccinated players if they don't have symptoms, then to pull this randomly. Doesn't make any sense to me.
Like Mayfield, Browns star defensive end Myles Garrett raised some questions about the league's testing policies.
Garrett said it was unusual that players weren't tested before entering the team's facility on Thursday, meaning those who were positive might have entered the building and increased the potential for spread.
"It almost seemed like we were just ignoring it instead of addressing the issue,'' he said. "We should go ahead and test and then allow in the building but not before you get results, and that is how it should be.
"The lack of testing that we have done all year I kind of feel like has led to this point. Only testing one time a week and guys doing their own thing on Monday and Tuesday with their time off, it was kind of a recipe for disaster.''
Schefter and Martin reported that four more Browns defenders, including safety Grant Delpit and cornerback A.J. Green, have tested positive, bringing the number in the protocols to about 20. The Browns already added safety Ronnie Harrison Jr. to the COVID-19 list Thursday.
With safety John Johnson placed on the COVID-19 list earlier this week and cornerback Greg Newsome ruled out because of a concussion, the Browns could be without their top three safeties and three of their top five cornerbacks if the players on the virus list aren't able to return by Saturday.
Earlier Thursday, the Browns activated tight end David Njoku from the COVID-19 list. Njoku, who missed last week's game against the Baltimore Ravens, rejoined the team in time to practice Thursday.
Coach Kevin Stefanski was away from Thursday's practice after testing positive earlier this week. If Stefanski is not able to return for Saturday's game, special teams coordinator Mike Priefer will serve as Cleveland's acting head coach.
The Associated Press contributed to this report.