The 21-year-old was signed by United from Swansea City on Wednesday.
"First of all, his best attribute is his speed," Giggs told United's official website.
I mean, I've probably not seen anyone as quick as him in my whole career.
"That's a big shout, because obviously I've played [with] and played against quick players."
James' national team manager also stated which three positions the talented youngster could play for United.
"He can play a number of positions – anywhere across the front three, really – and, this year, he's played quite a bit for Swansea up front, which he's not done a lot of in the past, but he's looked comfortable," he added.
"He's a nightmare for defenders. He's not that big, so it's hard to get your arm across him. He's both-footed, can score goals, create goals."