The 24-year-old was limped out of training with Brazil on Sunday, and could be forced into having surgery in order to recover from the injury
The 24-year-old was picked up the injury during training and was led off the pitch by medical staff, unable to walk due to a suspected ankle knock.
However, it has now been confirmed that he has broken his fifth metatarsal in his right foot, and may be forced to have surgery.
"I'll be back soon," he promised to reporters.
"There's plenty of time before the 2013 Confederations Cup, and I hope to play a part in it."
Jose Mourinho is already without the services of another left-back, Fabio Coentrao, who was forced out of Portugal's match with Russia in the first half on Friday, leaving him with a selection dilemma ahead of next week's clash with Celta Vigo.