Sandro was sent for a scan after complaining of pains in both his calves and his left knee in training during the country's Copa America campaign in Argentina.
Results showed that the midfielder had ruptured the meniscus in his knee, an injury which requires surgery and usually takes around a month to heal. That makes him a doubt for Spurs' season-opener against Everton on August 13.
The Brazilian Football Federation say that the 22-year-old will return to Tottenham, who will assess his injury further, and he will take no further part in the Copa America.
The news comes as a big blow for the combative midfielder, who was hoping to force his way into Mano Menezes' first XI after missing out on Brazil's two disappointing draws in the competition so far.
"It is a problem that I have had been feeling for some time," Sandro was quoted as saying on the UOL Esporte website.
"It has been troubling me more recently because it has stopped me practising.
"The examination found that I have to have surgery. Unfortunately for me the injury happened now, at an important moment of my career. But I will have the surgery and try to recover soon."
Brazil team doctor Rodrigo Lasmar said: "There is swelling muscle in the calves and a rupture of the meniscus externally in the left knee."
Sandro has become a popular figure at White Hart Lane since his move to the club in March 2010.
The Brazilian, who signed from Tottenham's feeder club Internacional for ?8million, played 26 times for Harry Redknapp's team last season, and made his international debut in 2009.
Source: Team Talk
Source: Team Talk