OWINGS MILLS, Md. -- The Baltimore Ravens' defense suddenly got thinner after the team placed four players, including Pro Bowl outside linebacker Justin Houston, on the reserve/COVID-19 list on Friday.

In addition to Houston, Baltimore put outside linebacker Jaylon Ferguson, nose tackle Brandon Williams and defensive tackle Justin Madubuike on the reserve/COVID-19 list.

Ferguson tested positive for the coronavirus on Friday, a source told ESPN. Houston, Williams and Madubuike were identified as high-risk close contacts, a source added.

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.

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.

For close contacts, the NFL policy states that unvaccinated players must stay away from the team for five days. Vaccinated players do not have to quarantine if they are a close contact.

Ravens coach John Harbaugh said the player vaccination rate was close to 90% at the start of training camp.

When the Ravens play at the Detroit Lions on Sunday, Baltimore will be down to two healthy outside linebackers (Odafe Oweh and Tyus Bowser) and three active defensive linemen (Calais Campbell, Justin Ellis and Broderick Washington).

On Friday, the Ravens ruled out defensive end Derek Wolfe (back/hip) and listed outside linebackers Pernell McPhee (shoulder) and Daelin Hayes (knee) as questionable.

With Houston out, Oweh is expected to make his first NFL start. Oweh, a rookie first-round pick, was named AFC Defensive Player of the Week after forcing and recovering a critical fumble in Sunday's 36-35 win over the Kansas City Chiefs.

The Ravens can help fill the void by calling up outside linebacker Chris Smith and defensive tackle Kahlil McKenzie from the practice squad.

Baltimore has given up the second-most yards in the league after two games. But the Ravens' defense stepped up late against the Chiefs, forcing turnovers on two of the final three drives.

In addition to a depleted defense, Baltimore will be without left tackle Ronnie Stanley (ankle) for a second straight game. Jackson (illness) is listed as questionable, but he's expected to start after being a full participant in Friday's practice.