Dutchman Verstappen's lead on Lando Norris in the drivers' championship was cut to 62 points after he finished sixth at Monza as his barren run of GPs without a win was extended to six.
And asked by reporters whether he was worried about losing what had looked like a certain fourth straight world crown when he won the Spanish GP in June, Verstappen said: "At the moment both championships (drivers' and constructors') are not realistic."
"The car is undriveable. It's a massive balance problem that we have. And that of course is not only over one lap but also in the race," said Verstappen.
He expressed his frustration with changes to his car which, combined with a series of improvements made by Norris' team McLaren, have left him and Red Bull looking over their shoulders.
With eight races remaining McLaren are just eight points behind Red Bull and on current form look favourites to win a first constructors' title since 1998.
"It's up to the team to come with a lot of changes with the car because we basically went from a very dominant car to an undriveable car in the space of, what, six to eight months," added Verstappen.
"We need to really turn the car upside down... With how we are at the moment, we are bad everywhere. So we need a lot of changes.
"Last year we had a great car, which was the most dominant car ever. And we basically turned it into a monster."
Red Bull have two weeks to make changes to their car before the Azerbaijan GP which will be held in Baku over September 13th-15th.
