In addition, the team also waived/injured quarterback Max Hall and tackle Jake Vermiglio. Hall, who started three games last season as a rookie, suffered a dislocated left shoulder in practice on Monday.
Croyle spent the past five years with the Kansas City Chiefs, who made him a third-round pick in the 2006 draft. He appeared in two games last season, starting one, and completed 8-of-19 passes for 38 yards with one interception.
The 28-year-old former Alabama star has made 10 NFL starts and played in eight other games. He appeared in nine games and started six during the 2007 season, throwing for 1,227 yards with six touchdowns and six interceptions.
Croyle could see action this Saturday when Arizona hosts San Diego in the team's third preseason game. John Skelton, expected to be the backup to new starter Kevin Kolb, is bothered by a sprained ankle that had him in a walking boot earlier this week.
Zastudil is a 10-year veteran who spent the past five seasons with Cleveland. He missed the entire 2010 campaign on injured reserve after being unable to sufficiently recover from a knee injury suffered the previous November.
In 113 NFL games for Cleveland and Baltimore, Zastudil has averaged 42.7 yards per punt.