The Ravens are undergoing contact tracing. If a player is unvaccinated and ruled a high-risk close contact, he must sit out at least five days and can return once he tests negative throughout.
If fully vaccinated, Ferguson would need two negative tests 24 hours apart to return, as long as he is asymptomatic. If unvaccinated, Ferguson is subject to a mandatory 10-day quarantine.
Ravens coach John Harbaugh declined to comment on any COVID-related matters after Friday's practice.
"We haven't put anybody on any COVID list at this point," Harbaugh said. "I'm not allowed to comment on it past that."
There were four other players who were absent from Friday's practice that hadn't been previously listed on the injury report: outside linebackers Justin Houston and Pernell McPhee, inside linebacker Chris Board and defensive tackle Justin Madubuike.
Meanwhile, quarterback Lamar Jackson, who missed Thursday's practice because of an illness, returned to action Friday.
Baltimore plays at the Detroit Lions on Sunday.
Ferguson, a backup who is in his third season, is the third Ravens player to test positive since the start of training camp in late July. Quarterback Lamar Jackson and running back Gus Edwards tested positive at the start of camp and quarantined for 10 days.
A third-round pick in the 2019 NFL draft, Ferguson has two tackles in two games this season (11 defensive snaps).
ESPN's Kevin Seifert contributed to this report.