The 24-year old Republic of Ireland international has attracted plenty of interest from other clubs after scoring 21 goals in the English Championship last term and McDermott says he may not be a Royals players next season.
"If a Premier League club comes in and we decide the deal is right and Shane wants to go there's nothing we can do," admitted McDermott.
"It's important I do the right thing by him as well. For me he'll go down as a very special Reading player.
"Do I want to lose Shane Long? No, absolutely not.
"Last season we lost Gylfi Sigurdsson just before the transfer window shut and it took us a while to get over that - 12 or so games."