The Danish international has missed the Bluebirds last two pre-season games against Brentford and Chievo and is to undergo a scan on his ankle problem.
Mackay admitted he is not sure yet if the 20-year-old will be ready for the beginning of the campaign.
"We're working on him to get him back for the start of the season," he said.
"I would be guessing right now if he will be fit for the first game but we still have two weeks."
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