The Bluebirds headed to Upton Park for their first-ever Premier League fixture in mid-August, and were on the end of a 2-0 defeat as they were out-played by their hosts.
"It was a disappointing result (at Upton Park), but the performance after the first 20 minutes was decent. This is a completely different game, a different week with different players on show," said Mackay.
"After the first 20-25 minutes at West Ham, we have got better, that showed in terms of our possession stats that day and also since then.
"The players have got stronger in their belief in terms of their ability to handle playing against top players.
"The Manchester City result strengthened belief and I would go even further and say the Everton result and the way we played was even more important. It could have been a case of after the Lord Mayor's Show. But it wasn't."
Mackay is set to rotate his squad for the trip to Upton Park. The likes of Nicky Maynard, Mark Hudson, Joe Lewis, Don Cowie, Jordan Mutch and Craig Noone are all in contention to feature, while Mackay could opt to hand new signing Peter Odemwingie a first start.
Odemwingie made an impact off the bench against Tottenham, and Mackay is full of praise for his final addition of the transfer window.
"He performed exactly as I thought," he said.
"He has got guile, he has great touch, awareness. he was causing Jan Vertonghen massive problems.
"I know exactly what I have got. I have an articulate, intelligent lad who has had a smile on his face since the day he got here. He is in need of match minutes and training.
"We have to bring him in gently into the group, but we have an exciting talent here for what I believe is a fantastic price.
"He will be involved with the squad on Tuesday, with the starting group from the Spurs game, we will look at to see how much they will be involved."