Rambo, 24, last played with the Washington Redskins before being released in September. He played 11 games in 2013, recording 31 tackles and one sack.
A sixth-round pick by the Redskins out of Gerogia in 2013, Rambo was released earlier this season when Brandon Meriweather returned from his suspension.
McKelvin was moved to injured reserve after fracturing his ankle last Thursday against the Dolphins. At the time of his injury, McKelvin led the team with four interceptions this season.
Buffalo (5-5) has lost two straight games and currently sits in third place in the AFC East. The Bills have home games against the Jets and Browns the next two weeks.
The Bills haven't made the postseason since 1999.
- Brad Rowland