In addition to trading for defensive end Lawrence Jackson, the Lions released two players: Marvin White and Matt Clapp.
Safety Marvin White joined the Lions last season and actually started six games. So far this training camp he has mostly been a depth player, and despite depth getting thin in the secondary, he was released today. Perhaps this is an indication that a move involving a safety is coming. (Just thinking out loud.)
Fullback Matt Clapp was an undrafted free agent signed out of Oklahoma. He was a long shot to make the team to begin with, as the Lions' depth at fullback is pretty good. Clapp had been injured the last couple of days, and the Lions decided to simply let him go.
Based on my count, the roster will stand at 79 players once Jackson officially joins. I assume this means another move is coming, but there's no telling if it will happen immediately, as earlier this week the Lions were at only 79 players for a couple days.