Lions linebacker Landon Johnson was placed on injured reserve on Thursday by the team. He suffered a concussion against the Miami Dolphins last week. Since he wasn't going to be able to play on Sunday against the Minnesota Vikings, putting him on IR allows the Lions to sign another player.
The "another player" in this case is cornerback Paul Pratt, who was on the practice squad. The Lions filled Johnson's spot on the roster with Pratt, who spent all of two days on the active roster back in October. The rest of the season he was on the practice squad.
Part of the reason the Lions decided to activate a defensive back stems from the fact that Chris Houston again missed practice on Thursday. He was out Wednesday as well, so right now it's looking like he may be doubtful for Sunday because of his shoulder injury.
Calvin Johnson also may be a bit doubtful at this point. He sat out Thursday's practice with a sprained ankle, and although he said that he hopes to play on Sunday, it's unclear if he will be able to at this point.
The only player who missed practice on Wednesday and was out there on Thursday is Tye Hill. He had an illness that kept him out of practice on Wednesday, but he was back on Thursday.