Leinart watched for 2½ seasons while Warner took the team to a level of success long-suffering Cardinals fans had never dared to even imagine: two NFC West championships, consecutive winning seasons and, of course, a Super Bowl.
In the few chances Leinart got to play, his performances often were erratic, and fans who once hailed him as a savior for the franchise turned on him.
Through it all, Leinart has stayed the good soldier, praising Warner, saying he would work hard and would be ready when he got his chance.
For now at least, Leinart has that chance. Whether Arizona makes any move to add someone else remains to be seen.
"We've got plenty of time to sort things out," coach Ken Whisenhunt said after Warner announced his retirement at a news conference on Friday. "Matt's not a rookie. He's been in this offense for three years. He's worked everyday with a guy that was playing at a high a level as anybody in the game and understands a lot more now about what he has to do to prepare in this offense."
Leinart has 17 starts in his four NFL seasons, but 16 came in his first two years. Then-coach Dennis Green benched Warner in favor of the rookie Leinart five games into the 2006 season.
When Whisenhunt took over the following season, Leinart remained the starter, although increasingly Warner was used when Arizona went to a no-huddle offense. When Leinart went down with a broken collarbone five games into the season, Warner took over.
Leinart has had one start since then -- this season against Tennessee when Warner was out with a concussion. Leinart played without the benefit of the practice reps a first-string quarterback normally gets because the decision to hold Warner out came at the last minute.
"What I think back to is the second half in the preseason when he threw for 300 yards," Whisenhunt said.
Of course, that was mostly against Packers reserves.
Warner said he knows he is leaving the team with a big hole to fill. He said many people -- "some of them in this room" -- tried to talk him in to returning.
"I think that was one of the hardest parts of the decision -- knowing an organization, a coaching staff, teammates, how they've counted on you, what a big part of the puzzle you are," he said. "There's no question that's the hardest part for me."
If Leinart is the quarterback, he will inherit the same talented receivers that caught so many passes from Warner, with one possible exception.
Anquan Boldin goes into the final year of his contract and, with his long-held desire for a bigger deal, a trade becomes a greater possibility.
But Arizona has something Leinart never had in his days as a starter, a sound and improving running game in Beanie Wells and Tim Hightower. Whisenhunt could well choose to reign in the offense and move toward a greater emphasis on the running game, a tactic he used in his days as offensive coordinator of the Pittsburgh Steelers.
Whatever the future holds for the Cardinals, message boards abound with jubilation from fans of the San Francisco 49ers and Seattle Seahawks, who see Arizona's days as the NFC West powerhouse coming to an end.