On the uphill romp towards Massenet, Red Bull driver Perez and Magnussen tangled wheels, sending Perez into a heavy crash that red-flagged the race.
The chasing Nico Hulkenberg also got caught up in the accident after being unable to avoid Perez's spinning Red Bull clipping his rear axle, ending the race of all three drivers on the spot.
"It was obviously stupid between Kevin and Checo," Hulkenberg told Sky Germany.
"I don't know if Checo saw Kevin. Checo could have left space. At the same time, I think Kevin was also very optimistic about staying on there, where the track gets narrower and there are little kinks up to Turn 3."
Expanding to the written press, the German said: "I think unnecessary from both really, it could have been avoided easily.
"Obviously, for me, who wasn't directly involved, it's the shittiest of all. I missed it probably by two tenths.
"If I would have been two times further up the road, he would have missed me. I'm gutted and disappointed because everyone in the team, we all invest a lot of energy and time and it's just frustrating."
Asked by Motorsport.com if he had sympathy for Magnussen going for such a small gap on a circuit where overtaking is extremely difficult, he replied: "Yeah, of course, but nobody has won the Monaco Grand Prix on lap one. So, you have to take calculated risk and risks that make sense...
"It's always that risk-reward question that you have to [answer] yourself."
Magnussen said he expected Perez to leave more space as the curve of the road meant the Dane's gap on the right-hand side was naturally starting to close.
"Well, he clearly wasn't leaving space that I thought he would," he argued. "I had a good part of my front end, my whole front wheel was ahead of his rear wheel.
"So, I did expect him to be leaving room for one car on his right, especially since he didn't have anyone on his inside. On his left, there was a completely clear track. So he just squeezed me into the wall."
"It's not good to see both cars in one crash. It sucks. It's a shitty, shitty situation."
Magnussen is just two penalty points away from a one-race ban but escaped sanction after the FIA stewards deemed the clash a racing incident, saying he "would have been very surprised" if he had been penalised for the contact.
It was an early end to a frustrating weekend for Haas, with both drivers demoted to the back of the grid because their rear wings failed inspection after qualifying.
Esteban Ocon was the fourth lap one retirement after clipping Alpine team-mate Pierre Gasly at Portier, with Ocon sustaining rear-end damage after bouncing back onto the tarmac. Gasly was able to take the restart with no major damage in 10th place.
