The New England Patriots have signed safety Michael Stone and offensive lineman Gene Mruczkowski. Stone will fill the open roster spot of safety Rodney Harrison, who was placed on injured reserve Tuesday because of torn knee ligaments. The 27-year-old Stone was a second-round pick of the Arizona Cardinals in 2001. He appeared in 37 games over four seasons with the Cardinals, playing mostly on special teams. Mruczkowski played in 10 games with the Patriots last season. He was released by the club earlier this month in the final cutdown. New England had a second open roster spot after linebacker Wesly Mallard was waived on Tuesday. The Patriots also made a practice squad change, releasing linebacker Jared Newberry and signing guard Ryan Krug.