McDermott has been given the go-ahead by chairman Salah Nooruddin to strengthen his squad, but has been unsuccessful in his bid to bring former striker Luciano Becchio back to the club, plus Arsenal midfielder Serge Gnabry in on loan.
Asked whether other targets could arrive in time for the weekend, McDermott said Premier League clubs were reluctant to release players at this stage of the season.
"I wouldn't have thought so," McDermott said after Tuesday night's 2-1 home win over Bournemouth ended a run of four straight defeats for his side.
"I went for two players and it's very difficult to get players from the Premier League.
"They've got squads of 25 and Premier League managers don't necessarily want to let one of those 25 go, for obvious reasons because they can't add (to their squads).
"I understand that and there's no point in us adding a player just for the sake of it - I won't do that.
"If we can get a player we feel can make a difference, then we'll do it, but if we don't, we won't."
McDermott also hinted he was reluctant to make another bid to bring Becchio, sold to Norwich in January in a deal that saw Steve Morison move in the opposite direction, back to Elland Road.
"Well he plays for Norwich and I understand Chris (Hughton's) situation now," added McDermott.
"He's one of his 25. He's got four strikers and if he loses one, then Luciano will be involved, so that's how it is."