The 21-year-old centre-back was a rock at the heart of the defence, making key interceptions, blocks, and clearances as Chiefs overcame a 10-man Stellies at a packed Athlone Stadium in Cape Town on Sunday evening.
Goals from Nkosingiphile Ngcobo, via a spectacular free kick, and Glody Makabi Lilepo from the penalty spot secured all three points for the visitors.
Nabi, however, was not present to witness the performance, having travelled to Tunisia on Friday after a serious accident involving his wife.
The club confirmed that he will return to South Africa once the family situation has stabilised. In his absence, assistants Khalil Ben Youssef and Cedric Kaze led the team from the touchline.
"Honestly, I don't have any words. I don't think I have a voice right now, but all glory to God," McCarthy told SuperSport TV after the game.
"This one is for my family - my mother, my father, my brother - and most importantly for coach Nabi. He couldn't be here with us today."
McCarthy, who previously captained Chiefs' DDC team, is following in the footsteps of his father, Fabian McCarthy, a former Chiefs and Bafana Bafana centre-back.
"I think it's all about God's timing. I've been with the first team for a while, but God knew the right moment for me to step up was now," he added.
"My father is proud, and I am too, but only God knows how hard I've worked for this opportunity."
For McCarthy, the performance was not just a personal milestone but also a statement of readiness as Chiefs aim for a strong start to their new campaign.