The Bluebirds had two great chances to hit the scoreboard when Fraizer Campbell attempted to round Spurs keeper Hugo Lloris, who may have handled outside the area, and Ben Turner headed home only for the goal to be disallowed for a foul on Lloris by Aron Gunnarsson.
Tottenham eventually won courtesy of Paulinho’s injury-time goal which prompted Mackay to wonder what might have been.
"Hugo Lloris will see it tonight and realise he is a lucky man and the referee will be disappointed in his decision," said the Cardiff boss.
"If you look at it again, it is Lloris who has his hand on Gunnarsson and then completely misses the track of the ball so for me it is a clear goal.
"The incident with Fraizer is one people will look at whether it was out or in, but we had a one-on-one and Fraizer is invariably brilliant at those, and he is more disappointed he did not skip by him. Whether it was a foul or not is open to question.
"We had some real good chances, but it's about being clinical."