Verstappen's frustration boiled over throughout both practice sessions as his upgraded Red Bull RB20 proved hard to control on the technical Italian venue's second sector.
The triple world champion ended first practice in fifth and slipped to seventh in the second, over half a second behind Ferrari's pacesetter Charles Leclerc. His team-mate Sergio Perez was next up in eighth another tenth behind the Dutchman.
Verstappen frequently vented his frustration over the RB20's recalcitrant behaviour and unpredictable balance on the team radio.
After the session, he feared Red Bull was too slow despite the new upgrades, which include an all-new front wing, a revised floor and rear bodywork tweaks.
"A difficult day. Just difficult to get a good balance and just not really feeling comfortable in the car, moving around a lot," Verstappen told F1 TV.
"It's very easy to lose the car, so we have a few things that we have to look at because today definitely.... just bad, just not comfortable.
"Also the long run was really bad, so definitely a few things that we have to improve if we want to be competitive tomorrow.
"Today we were just severely off the pace that we need to fix."
Asked if he had confidence the team would be able to turn it around for Saturday, he quipped: "I guess it can't be worse than what we had today, right?
"We'll look at a few things that we can do better for tomorrow. But yeah, it looks a bit like the others took a bit of a step forward. And from our side just a bad day.''
Verstappen's ire was also drawn by Lewis Hamilton, who impeded him through the Villeneuve chicane before Tosa. That led to Verstappen furiously gesticulating to the Briton as he pulled up alongside the Mercedes.
"It's not the first time," Verstappen said about Hamilton blocking him. "You try to, of course, always stay calm about it. But yeah, it happened again.
"But at the other end, I don't want to really talk about it too much, because that's not our issue today."
