The Magpies had lost four on the trot before Monday's game at St. James’ Park, but Maynor Figueroa’s sending off after 12 minutes helped Alan Pardew’s men to a comfortable victory.
And McArthur is looking for any negative fall-out from the loss to be eradicated with the visit of the relegation-threatened Rs next up.
"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."
Figueroa will serve his one-match ban for the clash against Harry Redknapp’s side - with Shaun Maloney, Ryo Miyaichi, Ivan Ramis and Ben Watson all likely to be ruled out through injury - but McArthur feels the club has enough quality to push up the table after the gap between themselves and the relegation zone was reduced to just two points.
"We need to work hard to get all three points and really push up the table," he added.
"We've got a good enough squad to deal with suspensions and injuries, and moments like these are dealt with because of the benefit of having a strong squad."