The 19-year-old Canadian has not played a competitive match since slipping awkwardly while facing Karolina Pliskova at the Queen's Club Championships in June.
After scans on her left knee, Mboko pulled out of her doubles quarter-final alongside 23-times Grand Slam champion Serena Williams and said she had injured the medial collateral ligament.
The injury forced Mboko to withdraw from the rest of the grass-court season, including Wimbledon.
Mboko said on Tuesday that the US Open tune-up event in Toronto meant "everything to her".
"Not being able to play at home this summer in front of the people I love and with the support of the Toronto crowd is deeply disappointing as it has been something I’ve been looking so much forward to," she said.
The Canadian Open runs from August 2nd to 13th in Toronto.