The Eels scored 24 points during their magic round clash with the Sydney Roosters, but leaked 31 to ultimately fall by seven points.
The loss follows their win last weekend against the Penrith Panthers, which came as something of a surprise, but that win followed a loss to the North Queensland Cowboys in Darwin where Arthur's side conceded 35 points.
Magic Round saw the Eels fall flat out of the starting gate, going into halftime 21 points to 6 down, before ultimately losing the game after a strong comeback effort.
Arthur said the club's defence simply isn't at the standard it needs to be when speaking at the post-game press conference.
“Our defence is not at the standard it needs to be at if we want to be serious about what we are doing,” Arthur said.
“It is just not good enough. You score 24 points it should be enough to win a game of footy.
“I’m not taking anything away from them because they are a quality football team, but if we score 24 points that should be good enough to win a game of football.
“The two elite teams at the moment, they start well and they do the same every week.
“That’s not conditional. They go after it every week physically and without question and we don’t at the moment.”
The comments come despite Parramatta getting the better of Penrith in last week's clash, which was Ivan Cleary's side's first loss at home in 21 starts.
Captain Clint Gutherson lambasted his side's start though, which saw them head into halftime with plenty to do.
“We were always confident we could get it back, but you can’t give a quality team like the Roosters that many points to kickstart their game,” Gutherson said.
“We are just disappointed in how we started that game.”
The difficult games don't stop there for the blue and gold, with a clash against traditional rivals and bogey team the Manly Sea Eagles ahead on Friday evening.
The Sea Eagles hold an 87-54 record over the Eels, while Parramatta have also lost all of their last three between the sides.