"Mom was crying," Darnold said Tuesday. "As usual."
His family will be in Detroit on Monday night, when Darnold will become the youngest opening-day quarterback in modern history -- 21 years, 97 days old.
"As a football player, whether it's Pop Warner, high school, college or now the NFL, your dream is to play the game," Darnold said. "That's what I want to do. I just want to go out and play every single day.
"If Josh got the nod, I was going to be pumped for him and be the best backup I could be. Now that I know I'm the starter, it's just an amazing feeling."
Darnold, who started only 22 games at USC, played beyond his years in the preseason, impressing the coaching staff and teammates with his maturity and poise in the pocket. It became apparent after the second game that he was trending toward the starting job.
As for being the youngest Week 1 starter, Darnold said it's "cool" and "awesome," but also a reminder.
"Knowing I'm young compared to a lot of other starting quarterbacks," he said, "it's ... I don't know. For me, I think it's just affirmation, knowing I have a long way to go."
Not surprisingly, McCown took the news in stride, displaying no ill will. He opened his news conference with a touch of gallows humors, saying he'd gladly take any questions about quarterback Davis Webb, whom the Jets signed to their practice squad.
McCown acknowledged he's "personally bummed," but he said Darnold proved he's ready.
"Obviously, as a competitor, you'd love to play, but I kind of understood which direction it was headed," said McCown, who will make $10 million in his backup role. "I'm excited for our team and excited for the future. ... We're backing it 100 percent."
Darnold's first assignment will be a challenging -- a prime-time game on the road, in a noisy indoor stadium. Coach Todd Bowles insisted he's not concerned by the rookie's lack of experience.
"He's been playing football his whole life," Bowles said. "He's not just coming fresh out of the womb and grabbing a ball for the first time. He's been in crowds, he's seen crowds and he's been playing football. I don't think it's that big of a deal."