Gardner-Johnson is feared to have suffered a torn pectoral during the 37-31 overtime loss to the Seattle Seahawks, a source confirmed to ESPN. NFL Network was the first to report the news.
Gardner-Johnson played 70 defensive snaps against Seattle, finishing with eight tackles and two stuffs.
Gardner-Johnson signed with the Lions as an unrestricted free agent on a one-year deal and was one of their biggest offseason additions to help the secondary, which was in dire need.
Houston, who had eight sacks last season when he was a rookie, has an injury to his fibula.
Both Gardner-Johnson and Houston will have to miss a minimum of four games after being placed on IR, although Gardner-Johnson's injury, if confirmed, could sideline him for the rest of the season.
A plethora of injuries have hit the Lions. Running back David Montgomery (thigh), is listed as day-to-day after being carted off against the Seahawks and told reporters it could take a "couple weeks" for his injury to heal. Left tackle Taylor Decker (ankle) missed Sunday's game, and right guard Halapoulivaati Vaitai (knee) is expected to miss significant time.