On August 9, 2011, Harrison signed with the Detroit Lions. Harrison was traded back to the Eagles in exchange for running back Ronnie Brown and a seventh-round draft pick in 2013, but the trade was voided the next day after Harrison failed his physical with the Eagles. On October 20, Adam Schefter of ESPN reported that Harrison failed the physical because doctors discovered a brain tumor. He had surgery, during which the tumor was discovered to be benign, but it was surrounded by blood vessels, which led to complications. He was placed on the reserve/non-football injury list by the Lions on October 21, ending his season.