The 18-year-old winger, eligible to represent the USA, was released by Southampton in the summer after progressing through their academy which he joined at the age of 10.
McDermott said: "He's only 18 and we kind of knew him already.
"He played in a couple of games and took part in some training sessions and he looks like he's got a chance.
"He's quite an exciting player - very, very quick. He's a great lad too. He really wanted to sign for us and he's been training with the first team. He looks good."
McDermott has been in the hunt for natural wide players since he arrived at Elland Road in February last year.