DENVER — The Giants made their most significant roster move of the season Sunday morning, designating for assignment struggling third baseman Casey McGehee, who had a terrible start to the season. The Giants are installing rookie Matt Duffy as their everyday third base and hope McGehee will accept an assignment to Triple-A, assuming he clears waivers. McGehee was emotional as he talked to reporters Sunday morning, after taking Duffy into a room in the visiting clubhouse at Coors Field to make sure Duffy knew that he deserved to be the everyday third baseman and should not feel bad about supplanting McGehee.