Cowell spoke with ET's Matt Cohen after Tuesday's finals, and he opened up about how he got in touch with Nightbirde.
"As you know, my Golden Buzzer, tonight, couldn't perform. But I did speak to her today," Cowell shared. "I said, 'I'm calling you because I know how you must be feeling.'"
During Tuesday's show, Cowell addressed the audience and revealed that Nightbirde's health battle is actually improving.
"We are missing Jane tonight, I spoke to her earlier on," Cowell told host Terry Crews. "The great news is she's doing better! I know she would have been here at the finals and we're all sending you, Jane, our love. We miss you. Just get better. We love you."
Cowell told ET that not only does he think Nightbirde would have made it to the finals, he feels she'd have had a real shot at going all the way.
"Would she have won tonight? I think she probably would, actually," Cowell shared with ET. "And I think she knows that deep down."
"Having said that, she knows she made the right decision," he added.
"Sharing my heart with the world on AGT has been an honor and a dream come true. My point of view this summer has been astounding," she shared in her post, alongside a somber black-and-white snapshot of herself sitting in her bed. "What a miracle that the pain I’ve walked through can be reworked into beauty that makes people all over the world open their eyes wider."
"Since my audition, my health has taken a turn for the worse and the fight with cancer is demanding all of my energy and attention," she continued. "I am so sad to announce that I won’t be able to continue forward on this season of AGT. Life doesn’t always give breaks to those that deserve it—but we knew that already."
Nightbirde concluded, "Thank you for all your support, it means the world to me. Stay with me, I’ll be better soon. I’m planning my future, not my legacy. Pretty beat up, but I’ve still got dreams."
The America's Got Talent finale results show -- in which one contestant will walked away with the $1 million grand prize and a special stage show at the Luxor hotel in Las Vegas -- kicked off Wednesday at 6 p.m. ET/PT on NBC.