Fox Sports first reported the news.
The Jets signed Harrell because backup Greg McElroy re-injured his knee in practice and won't play Thursday, Rapoport reported, per a source informed of the Jets' plans. McElroy's injury isn't serious.
Harrell joins a well-known but underwhelming quarterback group in New York. Mark Sanchez continues to recover from a shoulder injury and will not play in Thursday's preseason finale. Questions swirl around rookie Geno Smith's readiness to take on a starting NFL job. McElroy and Simms have been competing for the final quarterback job. Rapoport reported the Jets might keep all five quarterbacks on the roster for the time being.
Harrell lost the backup job in Green Bay to Vince Young last week. Much has been made of coach Mike McCarthy's backups, but after three seasons with the Packers, Harrell never blossomed. It's telling that McCarthy was willing to clip him in favor of a player not on a roster in 2012.
Harrell's signing is mostly about the Jets getting through their final preseason game, and he will have an uphill battle to make the roster beyond this week, even with McElroy injured.