McCarthy had Fletcher for two years at Wolves.
McCarthy told the Sunday Sun: "He has always been brilliant in the air. Steven is stronger than you think.
"Don't judge a book by its cover. Or rather don't judge a footballer just because he is skinny. Lionel Messi is hardly the biggest lad and he does okay for himself.
"Steven was very dedicated and spent a lot of time in the gym because he is not as naturally muscular as so many are. He was and is completely dedicated to his football."
Fletcher's first manager at Hibs was Tony Mowbray, another who eulogises him as a player and person.
The Middlesbrough boss said: "You look at Fletch's tattoos and you might think he is a certain type of character.
"However, he is a quiet guy and a good guy."
Fletcher's next test is next Sunday's derby. He is already a favourite with the Sunderland fans, but a winner against Newcastle will make him a real hero.
McCarthy said: "The thing about Steven is, he can only get better.
"He is 25 now and is still years away from his peak. The guy is a natural goalscorer and there are not too many of them about."