Following a chat with Ferguson, the on-loan midfielder opened the scoring as Watford secured their first win in almost two months with a 3-0 success over Sheffield United.
"Tom was with the Manchester United manager at the weekend, he went to the Emirates to experience what it would be like in that situation," revealed Mackay.
"He had five minutes with Sir Alex, who gave him targets for the season, so he's well on the way to meeting them.
"He's got a lot of potential and he's showing it here. It's about attitude and determination and Tom's got that."