The Scuderia have been considerably off the pace during the first few days at the Circuit de Catalunya and have been revealing very little. They did have an engine issue that halted their Friday progress, but they have completed far fewer laps than their rivals.
Verstappen admitted it's had to tell what direction his rivals are going in and hinted that they might have problems.
"They seem to have problems. Ferrari has not yet started a race simulation. That is why they are difficult to read," Verstappen told Auto Motor und Sport.
Red Bull aren't too far off
Red Bull were half a second off Mercedes during the first week of testing but the Dutchman remained calm and insisted they aren't as far off as it may look.
"We are not as far away as it seems.
"That is why we ensure reliability in the first place. Moreover, with a stable car you lay the foundation for the further development of the car. If you are reliable, you can develop aggressively during the season."