United won the first-leg of their Europa League semifinal 6-2, having at one stage been 2-1 behind.
Afterwards, Mariani said: "This is a direct collapse that finds no justification. I have always praised Roma's progress, but these 45 minutes have no justification. A too direct drop ... embarrassment, embarrassment, difficulty, they took the balance on the counterattack and then conceded incredible goals.
"In the second half it was a massacre from a sporting point of view, tonight's image is heavy and will also weigh on the balance of the two years of (coach Paulo) Fonseca."