"To try to replace a player like Shane would be nigh on impossible," McDermott told BBC Radio Berkshire.
"The most important thing is our group and I don't think we're going to find a player that will make the difference."
He added: "It's going to be about the team and how we're organised - how it's been the past two seasons."
West Ham have already had a bid for the Republic of Ireland international turned down, while West Bromwich Albion, Newcastle, Everton and Celtic have also been suggested as possible destinations for the 24-year-old.
Reading's Championship campaign begins in a week's time, at home to Millwall, and McDermott says he cannot be sure whether he will have Long available for that match.
"It's still very early days. I remember we lost Gylfi Sigurdsson I think three days before the window shut last season, so that's the sort of thing that could happen. But we'll have to wait and see."
Source: BBC Sport
Source: BBC Sport