The veteran actress, who plays shopkeeper Rita Sullivan on the ITV1 soap, is believed to have requested the time off during a meeting with producers on Thursday.
A Weatherfield insider told the Sunday Mirror: "Barbara felt bad that she had to go, but she explained it was in no way a serious health issue and that she'd be back.
"We had no option but to rewrite storylines and also shoot a scene in the Rovers Return to explain why she wouldn't be around for a while."
Knox, 75, made her debut as Rita in 1972, making her one of the programme's longest-serving cast members.
It is currently unclear how writers will explain the character's unexpected absence.