"Renault decided very late and a little unexpectedly in changing (Jolyon) Palmer," Marko told Auto Motor und Sport.
"Gasly cannot drive in Austin because our new engine partner Honda wants him to get the Super Formula title at the same time in Japan," he added. "We cannot deny their wish."
There is reportedly a huge list of potential one-off replacements for Gasly.
Among them are Robert Kubica, Sergey Sirotkin, Sebastien Buemi and even Takuma Sato and Indycar champion Josef Newgarden.
"Don't worry," Marko said. "We will know who is in the second Toro Rosso on Tuesday."
Buemi, a long-time Red Bull-backed driver, is perhaps the frontrunner.
But Marko admitted that bringing him back to F1 full-time next year as a replacement for Carlos Sainz is unlikely.
"Even if Toyota leaves Le Mans, it would be difficult to get Buemi next year because of Honda," Marko said, referring to the Swiss driver's existing contract. "The competition between the Japanese companies is too big."
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