And in Rotherham's battle at the bottom of the Championship, McCarthy, who brings his side to New York Stadium on Saturday, believes the duo are two men he'd have in the trenches.
“I think Neil Redfearn has gone back to them lads because when you come into a club you immediately think ‘who can I hang my hat on?’ and more importantly ‘who can’t I hang my hat on?’
"You find that out very quickly through the tittle-tattle in the dressing room and through the staff.
“You watch them in training and you find out about people very quickly. You want the players you can trust and Greeny and Frazer are certainly two tops guys, as they showed when they were here."
Redfearn has had a baptism of fire at the Millers following his appointment four weeks ago, having taken just one point from his opening five games at the helm and seeing his side slipping to five points from safety.
And McCarthy knows Redfearn is up against it.
“He has got his work cut out because that’s not an easy situation to be thrown into," he added.
"You’ve got to find out what your players are best at, find a system, find a shape and then they’ve got to work hard to stay in the league.