Verstappen has been courted by Mercedes in the past however committed to Red Bull back in 2017 through to the end of the 2020 season.
The Dutch talent has been one of the stand out drivers since joining the Formula 1 grid back in 2015 and seems destined for a world championship crown sooner rather than later. However, that championship triumph may come with another team as the Red Bull man could be on the move according to Marko.
"Of course I fear that Max will leave," he told Italy's Gazzetta dello Sport.
"At the moment there is only one winning car on the grid and that's Mercedes. We have taken a step and our performance is better, but we are still not fast enough to win races."
Marko also heavily criticised the change of Pirelli tyres for 2019, claiming that Mercedes was the only team on the grid benefitting from the reduction in tread depth.
"At the moment, Mercedes just has the biggest advantage from that stupid change of tyres. Nine teams are struggling with the rubber, one is not. In fact, that team even benefits from it.
"Previously the qualifying lap on Saturday was the most important, now the most important lap is the one before that. If you can't get the tyres to the right temperature, you can forget about your lap."
"Even if we are the fastest on a track now, a safety car will appear, the tyres will cool down and we will be the rabbit again. And whose idea was it to reduce the surface area of the tyres? Mercedes, because that team had constant problems with blistering. That's how you destroy the sport."