One of his toughest was letting him go.
Del Rio and the Jaguars said Friday they are parting ways with Leftwich after four frustrating, injury-filled seasons. They hope to trade him but are resigned to releasing the former first-round draft pick.
"To make a decision this bold requires some strong conviction, and I have that," Del Rio said.
Del Rio also chose mobile backup David Garrard as his starter heading into next week's season opener against Tennessee, saying Garrard gives the team a better chance to win.
"I believe David is our guy," Del Rio said. "He's proven to be a playmaker for us. He can be a threat both throwing and running it. We believe he's really improved during this offseason, and I feel he's earned this opportunity.
"He has a quick release, good feet and really has done a nice job of taking his game to another level," Del Rio said.
Garrard outplayed Leftwich in the preseason, completing 36-of-47 passes for 456 yards and a touchdown. He also ran for a score. But his numbers came primarily against second-team defenses.
"It's bittersweet news," said Garrard, who has started 18 games in five years as a backup in Jacksonville. "I'm definitely shocked. I didn't see this happening. I'm beyond shocked. I really feel like this is a dream, but I know it's not and I know I'm ready."
Leftwich, who missed 10 games last season because of an ankle injury, was 19-of-38 for 226 yards and a touchdown in three preseason games. He misfired on several passes, badly overthrew Dennis Northcutt on two deep routes against Green Bay and continued to show sloppy mechanics with his feet and throwing motion.
Making Del Rio's decision a bit easier, Leftwich hasn't played a full season since being the seventh overall selection in the 2003 draft. But the former Marshall star felt he was in the best shape of his life following ankle surgery and a rigid offseason conditioning program.
Del Rio seemed to agree, making Leftwich his starter in February and backing him every day since -- until now.
"Over the last five years, Byron was given every opportunity to lead this football team," Del Rio said. "We've gone through the entire offseason, training camp, preseason, and during that time, it became very apparent to me that David was the best choice for us, the best selection for us to lead this football team."
Leftwich is in the final year of his contract, so releasing him would not cost the team anything extra against the salary cap in 2007.
But it would leave the Jags with little room for injury at that position. Third-stringer Quinn Gray, who played well in the preseason finale after missing most of camp because of a sprained ankle, will move into the No. 2 spot.
The Jaguars are also trying to acquire a veteran quarterback to serve as a backup and may turn to former Jacksonville QB Mark Brunell.
"I'm comfortable with the two quarterbacks we have right now," Del Rio said.
Getting rid of Leftwich will end a somewhat turbulent four-year relationship with Del Rio. Leftwich openly questioned Del Rio's decisions, and Del Rio felt the quarterback lied to him several times about his health.
"This was a football decision," Del Rio said. "Any of those types of peripheral issues only helps or hinder, but they're not at the root of what this decision is about."
Leftwich has completed nearly 59 percent of his passes for 9,042 yards, with 51 touchdowns and 36 interceptions. He started 13 games as a rookie and then missed two games in 2004 with an injured left knee.
He missed five games the following year because of his oft-injured left ankle, the same one that cost him most of last season.
Garrard, 10-8 as a starter, has completed 58 percent of his passes for 3,543 yards, with 18 touchdowns and 13 interceptions.
"I'm sure it will be tough for a lot of guys," Garrard said. "The timing of it was really weird. ... It was tough for me, too. But I've been given the opportunity to lead this team and I want to make sure I do it."
Information from ESPN's Chris Mortensen and The Associated Press was used in this report.