First things first, the Lions waived Leger Douzable in order to make room on the roster for Ndamukong Suh. Douzable just joined the team rather recently and is a defensive tackle, so that move isn't really too surprising.
What is a little surprising is that the Lions released safety Marquand Manuel and signed undrafted free agent Randy Phillips, who was supposed to originally join the team back in April after the draft. Phillips has a lot of potential, but it's not often you see a veteran like Manuel released in favor of someone like Phillips, who really only practiced with the Lions during their minicamp as a tryout. I guess the coaches felt that Manuel did not have what it takes to make the roster, so they decided to see what Phillips will bring to the table at safety.