"It's great to see him back. He's been out an awful long time out and it was such an innocuous incident in training," Mackay told the Watford Observer.
"I've seen him come in and out of the physio room every day, be apart from the group and do stuff on his own.
"He's had some very dark times and it's great to see him back and back training.
"The physios have had a plan for him over the summer and he'll start training from day one and try and force his way in.
"He's got a fantastic touch and has great ability on the ball. It will be great to see him pull on a first-team jersey."