Montolivo will play no part in the FIFA World Cup in Brazil after fracturing his shin bone in Saturday's 0-0 draw at Craven Cottage.
Alberto Aquilani was also withdrawn just before the half-time interval to give Prandelli plenty to ponder as he looks to whittle down his squad to 23.
Italy were far from their best in London, though Prandelli put that down to the shock of seeing Montolivo pick up his serious knock.
"Unfortunately this match was marred by these injuries," he said.
"At the moment we are dazed, it isn't easy to absorb this blow.
"We were playing well enough at the start of the match, with a good numerical superiority.
"At the end of the first half it was difficult to talk about the match considering the injuries to Montolivo and Aquilani.
"We will evaluate the conditions of all, someone is a little fatigued, we will decide whether to bring one or more players."
