The number of F1 experts who believe Ferrari can win the 2019 world championship is dwindling.
"Barcelona is a circuit that tells you the truth," he said. "If you go badly there, you are not going to be able to compete for the world championship, even if from the interviews it seems that everything is going well."
Former Williams driver, and current Renault reserve Sergey Sirotkin, agrees.
"There are circuits on which Ferrari will be fast," he told Sport-Express. "But in general, they will not fight for the championship."
Red Bull boss Christian Horner said after Barcelona that 2019 looks like "a Mercedes championship", but he also thinks the 21-race schedule is "very long".
"Basically anyone can be beaten," he told Speed Week. "No run of success lasts forever.
"We have all the skills and the tools we need to beat them, and that's what we are trying to do."