Masi was taken off after 10 minutes of Italy's 32-10 reverse at Twickenham, adding his name to a mounting injury list of Azzurri backs and creating a selection dilemma for Brunel whose options at centre were already depleted.
"Unfortunately Masi broke his Achilles tendon. This is the third centre we have lost in two weeks so it is a series of events that leaves us with a bitter taste in the mouth," Italy's French coach said.
"We had injuries to three centres, Luca Morisi first of all, then we had an injury to Gonzalo Garcia, he is with us but is injured and I am not sure if he can play against Canada.
"Today Masi. We really need to analyse the situation because we don't have a lot of resources... possibly we will have to call someone in."
The injury to Masi effectively rubbed salt into the wound for Brunel, whose side had been outplayed against France, with their indiscipline handing their opponents a series of first-half penalties that left them trailing 15-3 at the interval.
South African referee Craig Joubert was quick to penalise the Italian scrum with a string of infringements prompting a quizzical reaction from the Italy coach.
"We didn't really understand the referee and the way he took his decisions led to the imbalance of the game," Brunel said.
"For a very long time we had no issue, we've had 12 scrums in all and we've been penalised six times and five on our put-in. I think that's quite a lot really."
An early France try after the interval opened up a 25-3 lead before a try from Giovanbattista Venditti stirred faint Italian hopes of a fightback.
That failed to materialise, however, and a second French try from Nicolas Mas hammered a final nail into their brief resurgence and left them needing an improbable win against Ireland to stand a realistic chance of making out of Pool D.
Captain Leonardo Ghiraldini clutched at a handful of positives that they could take into their next game against Canada at Leeds's Elland Road next Sunday (NZT).
"We played well at times, particularly in defence," he said.
"When we had the ball we managed to push forward... we have to balance, we have to continue, we have to grow. We must continue to work, we must continue the same level of intensity."
