The Black Cats go into today's encounter on the back of a shocking sequence that has seen them win just one of their last 14 Premier League matches.
The run includes being hit for seven at Chelsea and the concession of two or more goals to Birmingham, West Ham, Tottenham, Aston Villa, Manchester City, Blackburn and Everton.
Defensively, Sunderland have been suspect on a number of occasions, and McCartney admits the last three months have had a detrimental effect on morale.
I think it's fair to say that when you go through a run of defeats or without winning, then confidence will not be as high as it should be, said the full-back, who has missed just one of Sunderland's last 12 league games.
That's football there will be ups and downs throughout a season and it's up to us to get out of this rut we are in by starting to win games. We have another home game now, and that's the best place to turn things around.
Sunderland are in the middle of a three-game run against sides in the bottom half of the table that manager Steve Bruce predicted would define their campaign.
Monday's goalless draw with Stoke City was hardly the best way to start, and with a trip to Portsmouth following today's game with Wigan, Mc- Cartney admits it is imperative that the Black Cats start to turn things around soon.
We had such a good start, and all the talk was of maybe pushing for a place in Europe,
he said. Now we've gone so long without a win, people are saying that these three games Stoke, Wigan and away at Portsmouth are really big.
He (Bruce) says it's up to us to show what we are made of, and if we can take seven points from nine it would have been nice to take all nine then that would push us right up the table again.
You don't want to be dragged into the position we were in last year, where we were fighting against relegation despite a good start.
Everyone in the dressing room knows what this squad is capable of because of that start. But we have to make sure we get back to the level of performances that earned us wins against Liverpool and Arsenal.