"We have to decide on our future engine partner then," he said.
A partnership with Mercedes has already been ruled out, and becoming a customer of another arch-rival, Ferrari, must also be seen as highly unlikely.
Honda is returning to F1 as McLaren's works partner next year, with Red Bull magnate Dietrich Mateschitz recently saying he did not want to simply be a "customer" of the Japanese manufacturer.
There might be another way.
Red Bull already has close commercial ties to Volkswagen, the German carmaker giant.
"Volkswagen is already in other racing series, with engines that are similar to formula one technology," said Marko.