The three-time champion and series leader said on Thursday that he felt both Ferrari and McLaren could threaten his hopes of a third victory in the Mediterranean principality in Sunday's race.
"It's one of the more difficult tracks for us," said the 26-year-old Dutchman, who won last Sunday's Emilia Romagna Grand Prix by less than a second ahead of McLaren’s Lando Norris.
"In the last few years, Ferrari have always been very, very strong here and McLaren, lately, in the last two races, have really ramped up their performance so they are, for sure, ones to watch as well.
"In Monaco, you can always have surprises. Last year, Esteban (Ocon) did an amazing lap in qualifying so these kind of things can happen when someone feels really good, really confident."
Verstappen leads the drivers’ championship by 48 points ahead of Charles Leclerc of Ferrari, after winning five of the opening seven races this year, but knows that Norris, who claimed his maiden victory in Miami three weeks ago, has the speed to beat him.
The Dutchman hopes to extend his Ayrton Senna record-equalling sequence of eight pole positions to an outright new record of nine in Saturday's qualifying session that is so critical to the outcome of Sunday’s race.
The high kerbs around the barrier-lined street track represent a major worry to Red Bull, as Verstappen admitted.
Last year’s Singapore Grand Prix, on another street circuit with high kerbs, was the only event at which they failed to score a podium finish.
"Our car normally struggles a bit over bumps and kerbs," said Verstappen.
"We did work on it a bit, compared to last year, and I think our low-speed performance has improved a little on most of the tracks we've been to this year.
"But I don’t think it’s going to be an easy weekend! Monaco is never straightforward, even when you have the best car. It is so very tricky. You need to get everything to work and there is always a lot of disruption."
He added that he wanted to avoid a repeat of his experience at Imola where after a bad start, in two difficult days of practice, he recovered to win by 0.725 seconds, resisting Norris's late charge.
"For sure, things have closed up a lot since Miami and now it is very dependent on finding the best set-up. We have to be at our best and maybe in the last few races we haven’t been operating at that level."
