Crystal Palace fell to a disappointing defeat as they were beaten 3-1 by rivals Crystal Palace.
Palace went into the game on the back of an impressive 2-0 win against Burnley, but were brought back down to earth with a bump despite their opponents playing over 60 minutes with 10 men.
"It was not good enough, especially in the first half," McArthur told reporters after the gane.
"To give them three goals is not acceptable.
"Obviously the manager was not happy. Everyone knows it is not acceptable, performance or result wise. Even in the second half when we were pushing we did not put their goalkeeper under enough pressure when they have got 10 men. A poor performance all round and not acceptable."
McArthur gave away a penalty during the game, which Glenn Murray converted.
"I have 100 per cent got the ball," he said of his tackle on Brighton's Jose Izquierdo. "It has never been a penalty but he gave it."