Silva, 42, was removed from UFC Fight Night in Shanghai due to a potential doping violation. The UFC found an unlikely replacement in Michael Bisping, who is just 12 days removed from losing his title at UFC 217.
That switch between Silva and Bisping happened quickly. But to Gastelum, it felt like forever.
"I had to go through a full day of anxiety and nerves," Gastelum told ESPN. "I felt sick to my stomach.
"All these thoughts went through my head. I thought the event was going to be canceled. I was even asking my manager to get me on the Detroit card in December."
Ultimately, Gastelum (13-3) is content with how it played out. Although he admits this likely means he'll never fight Silva.
There's irony in that, considering Silva was the only reason Gastelum agreed to stay at middleweight earlier this year. The 26-year-old has struggled to make the 170-pound welterweight limit in the past but still sees himself returning to that division.
"Anderson was the only reason I even thought about staying at 185," Gastelum said. "If there wouldn't have been that, I would have tried to convince Dana to give me a fight at 170. I think that's where I'll end up.
"If I end up fighting for a belt at 185, then so be it. And my manager told me, by taking this fight against Bisping, I may have messed up and I'll get stuck staying in this division. But I feel like this might be my last fight at 185."
Regardless of which division Gastelum, who fights out of California's Kings MMA, competes in next, a finish over Bisping would be a nice feather in the cap.
During that 24-hour timeframe in which he didn't know what was going to happen, Gastelum says it never crossed his mind Bisping would take the fight -- but, of course, he's glad he did.
"I told [manager Ali Abdelaziz], the only guy that made sense to me was probably Derek Brunson," Gastelum said. "Not that I really wanted to fight Brunson, or wouldn't fight him, but it's just the one that made sense. He told me to wait.
"I feel like this is going to be an even tougher fight, honestly. Bisping is as tough as they come. I feel like he is a legend and a future Hall of Famer, and it's an awesome opportunity to face him."