Fast forward to today and the Lions have in fact brought another cornerback aboard. According to the Lions' official site, they agreed to a one-year deal with cornerback Justin Miller this afternoon.
Miller hasn't even appeared in an NFL game since 2009. He appeared in one game for the New York Jets that season and one game for the Oakland Raiders. In general, he hasn't spent a ton of time on the field for games since his first two years in the league (2005 and 2006). He did make the Pro Bowl in 2006 as a kick returner, but he's only been active for 12 games since 2007.
Miller will be thrown into the competition at cornerback, but based on his numbers and the fact that he hasn't even played in a few years, it would be surprising if he sticks around on a permanent basis.
It's not currently known who was released to make room on the roster for Miller.
UPDATE: The Lions have released wide receiver Marcus Harris, according to Tim Twentyman.