The red card knocked the stuffing out of Wigan as Ba went on to net a double and now they need to fight back when they meet bottom-of-the-table QPR at the DW Stadium.
"The red card had a big impact on the game and then of course they score a goal from the spot which is a double blow," McArthur told the club's official website.
"It was disappointing but we've got a massive match against QPR at the weekend and that has to be the main focus from now on.
"It is the biggest game of the season now in terms of trying to show people how we can battle against adversity like we did last season.
"We need to draw a line in the sand because we've played with 10 men for 80 minutes."