Murray, three times a winner of the Rogers Cup, will miss the event for the second straight year as he continues to deal with a troublesome hip.
“I am sad to be missing the tournament in Montreal because I have many great memories from my time in Canada. I am doing everything I can to return as quickly as possible” said Murray in a statement released by Tennis Canada.
Cilic ruled himself out with an abductor injury he says he suffered in the Wimbledon final against Roger Federer.
Both Murray and Cilic will now be in a race to be ready for the season's final grand slam, the U.S. Open, which begins on Aug. 28 in Flushing Meadows.
Murray claimed the title in 2012 while Cilic hoisted the U.S. Open trophy in 2014.
(Reporting by Steve Keating in Toronto. Editing by Toby Davis)