The midfielder suffered a knee problem after celebrating a goal during a session at the club's Harlington base last week.
Sandro, who was forced to miss Saturday's 2-2 draw with Stoke City, is now facing a lengthy spell out of action.
"It was a stupid injury. He scored a goal in training and slid on his knee like a lot of players do. I don't know why players do it," Redknapp is quoted as saying by Sky Sports News.
"The pitch was bone dry so he skidded. His knee has gone bang into the turf and I watched him trying to get up wondering 'what's he done here?' Sure enough, he wasn't fit for Saturday."
QPR continue their Premier League campaign this Saturday with a trip to Redknapp's former club Southampton.