The Irish striker has scored 11 goals for the Royals this season, his best return for the club with almost half the npower Championship campaign remaining.
Speaking to the club's official website, McDermott said: "Shane is coming of age. All the unselfish work he does, he's getting his rewards now.
"The way he lives his life, the way he conducts himself. He's a great guy."
Long's partnership with forward Noel Hunt is a key part of the side's success, with Hunt setting up Long's goal in the 1-0 FA Cup third round victory over West Brom.
McDermott added: "They're working really, really hard. They're trying to create things, they've got quality."