Mackay admits the 17 year-old has been "brave" in moving south from Rangers.
"He's got a lot of potential," he said. "He's been on our radar since he was 11 and Craig Mulholland and the academy coaches at Rangers have given him a fabulous grounding.
"He's now taken another step - which is a very brave step to take, going down to Chelsea - and he's hit the ground running."
But does Mackay think Gilmour has what it takes to make it really big on the world stage?
"He absolutely has got fabulous technical ability," he added. "It's his mentality. Can they have talent? Fine. Plenty of kids have that. But to play at the highest level, you need temperament.
"And we're certainly seeing that at the moment. It's potential and it's a progression, but it's a progression of potentially a very, very good player."