The midfielder was substituted just after the hour in QPR's 1-0 defeat at St James' Park, and scans have shown ligament damage.
QPR's club doctor Peter Florida commented: "Sandro took a bad knock to his right knee, and has suffered damage to his medial ligaments.
"It is totally unrelated to his previous ACL [anterior cruciate ligament] injury.
"Unfortunately, he will now be unavailable for the selection until after Christmas."
Sandro, who was plagued by knee injuries at former club Tottenham, has already overcome a similar problem earlier this season after sliding while celebrating a goal during training in September.
The 25-year-old, who arrived at Loftus Road in September, is set to miss QPR's Premier League fixtures against fellow strugglers Leicester City and Burnley.
