The world No. 3-ranked Azarenka lost to Samantha Stosur in the singles final at Carlsbad Sunday and made the decision to pull out of the Toronto event after meeting with doctors shortly after her match.
"I'm so disappointed that I can't make it to Toronto as I added it to my schedule knowing what an important event it is," Azarenka said. "I am sorry to all my Canadian fans and I will work hard to get healthy and back on the courts as soon as possible."
Azarenka was the No. 2 seed in the Toronto draw, meaning Caroline Wozniacki, the next highest seeded player, will move into Azarenka's position.
Wozniacki was originally slated to play Monday night, but Azarenka's withdrawal gives the 23-year-old a first-round bye.