The Bluebirds were initially in negotiations to sign the midfielder after agreeing a fee with Toulouse but the 41-year-old says the France international will now join Spurs
The Bluebirds were thought to have been favourites to win the signature of the France international after agreeing a fee for the 25-year-old in July and entering negotiations for personal terms.
However, Toulouse president Olivier Sadran revealed on Wednesday that the player will join Spurs and Mackay confirmed the news when asked about the deal to sign the midfielder.
"We had a bid accepted but it looks as if he has decided, as far as I am aware, to go to Tottenham. It looks like it will go that way," Mackay told reporters.
Despite missing out on Capoue, Mackay is refusing to be downhearted and says he is keen to press on with further additions following the club record deals that brought in Andreas Cornelius and Steven Caulker.
He continued: "With the targets we want, invariably there is also competition or clubs don't want to sell.
"We signed five in the last week of the window last year and we have brought in around 25 players to the club over the last two years, so we are well versed in the mechanics of how this works and the patience you sometimes have to have.
"Sometimes things happen quickly, like the Steven Caulker deal, other times it can be more protracted.
"We have some plates spinning at the moment, if it will take until September 2, then that is what it will take."