Cap Hit AAV would be calculated pro rata after the season starts - it's why Willy's cap hit last year was over $10,000,000 but his actual AAV is just under 7 million. Since we're so close to the salary cap this year after factoring in Marner's likely contract, we would exceed the cap if he waited like Nylander to sign a deal.
That doesn't mean it's impossible though. If Marner holds out and then has a change of heart and wants to sign after the season starts, Dubas could always trade away guys and their cap hits. It's not a likely scenario IMO, but this isn't exactly a "paint by numbers" negotiation.