The 34-year-old got his first taste of international football since before 2010 after coming on as a 33rd minute substitute for Dino Ndlovu in the 1-0 defeat against Brazil on Friday.
However, the Orlando Pirates striker soon needed to be subbed himself after injuring his hamstring just before the half-time whistle in Sao Paulo.
"I was very disappointed," McCarthy told The Sowetan. "And you know what, I was feeling so nice, confident and strong. But I think 34 is getting a bit heavy now - you know the body is starting to get more delicate than previously.
"I'm just disappointed that I didn't get to express myself because I think it was an amazing opportunity for us. If there was ever a time to beat Brazil, this was it."
The injury is also set to keep South Africa's all-time record goalscorer out of their second friendly international against Mozambique on Tuesday.