A good second half performanace saw them head back to the Midlands with a 3-2 win and all three points.
"I wasn't happy with the tosh we served up 20 minutes before half-time. Having dominated for the first 20 or 25 minutes we let them back into it," McCarthy said after the match.
"I was annoyed at half-time, but calm enough to know it was a game we could win given how we'd started the match.
"That end obviously wasn't good for goalkeepers as we were very fortunate with the winner.
"But you have to play to the conditions and it was always going to be scrappy given the surface.
"I was hard on the lads at the end because we won and they were all slapping each other's backs but we made it harder than it should have been to get back into it."