Black Cats boss Martin O'Neill called his old friend before the deal went through and asked for his opinion on the Scottish striker.
McCarthy told the Sunday Sun: "Martin had left Celtic by the time Steven made the breakthrough up in Scotland with Hibs, so he really did not know him as a person.
"He was also out of football by the time Steven made a name for himself with Burnley and then at Wolves with me, so he wanted to ask me a few questions about the lad in terms of him as a player.
"Martin asked for my opinion and I gave it. I will be honest with you; I waxed lyrically about Fletch.
"All I told Martin was that he was a brilliant professional and a great person.
"The lad was a joy to work with. I have not a bad word to say about him."