The 21-year-old shot-stopper kept his second clean sheet in the space of four days as the Hornets sealed a 1-0 win against the Championship strugglers at Home Park.
Mackay told the London Evening Standard: "He made two or three world-class saves and I don't say that lightly.
"There were a couple of reaction saves that were just immense and that's why he's England Under-21 goalkeeper at the moment."
Mackay also hailed the contribution of on-loan Manchester United midfielder Tom Cleverley, who bagged the only goal, and added: "We did our homework with him and he's settled immediately. He's a great young character.
"He's enthusiastic and he's settled into the group within five minutes, he's scored goals and he's set goals. Tom has shown a real willingness to be a team player."