Alex Albon appears to be in pole position to remain at Red Bull in 2020.
But Marko added: "He has some speed to gain in the race. He takes too long to get going."
Team boss Christian Horner, though, is more upbeat about Albon's chances of keeping the race seat for 2020.
He said the 23-year-old did an "amazing job" at Suzuka.
"He's certainly making a really good play for it," Horner told Channel 4. "It's his seat to lose in many respects and I think he's done a super job so far."
However, he warned that Red Bull will not rush to make it decision.
"We've still got plenty of time. All the drivers are under contract, we don't need to rush, but I'm very pleased with what he's done so far," Horner said.