Johnson was signed following a tryout that also included former Jacksonville Jaguars cornerback Rod Issac and former Atlanta Falcons cornerback Terrence Johnson, according to a league source with knowledge of the situation.
To create a roster spot for Johnson, the Ravens placed rookie running back Bobby Rainey on injured reserve with a previously undisclosed knee injury.
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Johnson, 33, has 29 career starts, intercepting eight passes with 170 tackles and three forced fumbles.
Ravens coach John Harbaugh indicated that the team would have to take a "hard look" at Smith's status after his groin failed to loosen up Sunday during warmups and at halftime. Although active for the 55-20 win over the Raiders, Smith didn't play as Corey Graham started at cornerback with Chykie Brown operating as the nickel back.
Besides Smith's injury, the Ravens are already missing shutdown cornerback Lardarius Webb for the remainder of the season due to a torn left anterior cruciate ligament.
Johnson was granted time off by the Raiders late last season when he was placed on the reserve-non-football injury list.
His sister was killed by a former boyfriend, and his mother was shot and made it through the incident.
Rainey is an undrafted rookie from Western Kentucky who was on the active roster for three games prior to his season ending with this knee injury.