Flowers will immediately fill the role Ansah was supposed to play.
Flowers is a former fourth-round pick out of Arkansas who became an impressive mainstay on the New England defense over the last few seasons, the first three years of which came with Patricia as the defensive coordinator.
Amendola, a 33-year-old slot receiver, is entering his 11th National Football League season.
Flowers is one of the top free agents on the market and is expected to get a contract in the $17-19 million per season range after four seasons in New England.
Detroit is led by general manager Bob Quinn and coach Matt Patricia, both of whom worked for the Patriots, and that clearly made Amendola comfortable joining the franchise.
The Lions were desperate for a pass-rusher, and Flowers was arguably the best one available in free agency. That according to the NFL Network's Ian Rapoport. He also started the last three Super Bowls for the Pats, winning two. Harris is a second-year player on the team's practice squad.
Monday's signings can not be official until the NFL's free agency period formally begins at 4 p.m. Wednesday, but teams are permitted now to work out deals with players that have been cut or whose contracts expired at noon Monday.