eli started 7 games during his rookie season; ben started 13 games. rivers only played in 2 games in relief in 2004 and 2005. he made his starting debut to open the 2006 season.. and has not missed a start since.
ben replaced xfl alumni tommy maddox, after an injury, while manning replaced a supposedly-ineffective kurt warner.
Ben went 13-0 when Tommy Maddox got hurt in the second game against Baltimore(Ben finished the game for a non-record, and Maddox played the last game against the Bills)
Rivers sat as Brees suddenly had a great season and started the career he has now
Eli Manning started as a backup if I remember correctly but did start toward the middle of the season.