The worlds No. 2 player said he planned to compete in the event for the first time since 1994, but never officially entered.
Mickelson will make his return in Ireland at the WGC-American Express Championship Sept. 19-22, the week before the Ryder Cup in England.
Mickelson was to be the marquee player at the Canadian Open, contested at Angus Glen Golf Club in Markham, Ontario. World No. 1 Tiger Woods, who won this event in 2000, has not committed.
Players have until Friday at 5:00 PM to enter.
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