Williams was placed on the list after coming in close contact with someone who has COVID-19, a source confirmed to ESPN. He did not test positive, the source said.
The plans for Baltimore haven't changed. The Ravens traveled to Philadelphia on Saturday afternoon.
Williams did not practice Thursday and Friday for non-injury related reasons. After Friday's practice, coach John Harbaugh said Williams had "a personal excused absence."
On Thursday and Friday, Ravens players were spotted wearing masks while walking out to the practice field, which is not something they previously did.
When asked a little over two weeks ago about what it will take to avoid COVID-19, Williams said: "Do as much as you possibly can to protect you and the family and the people that you love. Sometimes if you catch it, you catch it. Sometimes, it just is what it is and it's inevitable."
Williams becomes only the second Ravens player to be put on the reserve/COVID-19 list and the first since training camp officially started.
The Ravens' defense currently leads the NFL in fewest points allowed, giving up 15.2 points per game.