We were getting over-run before he came on, after he came on we were back in the game.
He's a water carrier, but a very good one. His job is to play the way he is facing, retain possession, offer himself to team-mates on the ball and hold his position to shield against opposition attacks.
This job is not as easy as it sounds, it's not about chasing around diving into tackles, it's about concentration.
Take Jack Rodwell, it is almost certain that this is the job Mancini bought him for (at 15m), he should be able to do it in his sleep but he doesn't have the head for it.
When McCarthy has a run in the team people probably will still ask the same questions but when he's out injured or suspended for a couple of games, THEN you will know his value.