San Jose Earthquakes defender Attakora, 24, was the last man added to Colin Miller's 23-player roster for the biennial tournament, but he never made it to Canada's training camp as he continues to battle with a concussion suffered in a club match in May.
D.C. United's Porter will take Attakora's place on the roster. The 23-year-old, who can play on the wing or in midfield, has one goal and one assist in 15 MLS games this year. He will will not immediately join the Canadian team, instead being placed on stand-by in the event that he is needed.
Canada opens the Gold Cup on Sunday in Los Angeles against Martinique.