Montolivo, whose country face England in the group stages in Brazil, suffered a fractured shinbone against the Republic of Ireland on Saturday night.
De Rossi expressed regret that his compatriot will not travel to the tournament. "In football there is nothing uglier than an injury," said the Roma midfielder.
It is a feeling which has rocked the rest of the Italy squad. "Once that happened, my players were affected psychologically," said Cesare Prandelli, the Italy manager.