The former Parramatta mentor saw his woeful Rhinos thrashed 38-0 by Wigan at the Brick Community Stadium on Friday.
Leeds' heavy defeat means they now face the daunting task of heading to second-placed Hull KR knowing they will need a win to stand any chance of avoiding a second straight campaign without a post-season berth.
The hosts' seven-try rout means they will confirm first place in the Super League table - and the most favourable route in the play-offs - if they beat Salford in their final match of the regular season next Thursday.
Tries from Jake Wardle and Australian Bevan French swept the hosts to a double-figure advantage with less than eight minutes gone.
Five more tries - including a second for former Eel French just before the final hooter - maintained Wigan's two-point lead at the top of the table, and left any realistic hopes the Rhinos harboured hanging by a thread.
Arthur admitted his team was beaten by a way better side.
"They dominated us physically right from the start, and from one to 17 across the field they owned their jobs and we didn't.
"We didn't handle their physicality right from the start and we never got a field position all night. In the first half we didn't have one set that started in their half, and in the second half they didn't improve much.
"We need to make sure it's a one-off. It's certainly not the way we want to finish our season."
In other matches, St Helens stepped up their play-off bid with a 40-4 win over Castleford while Hull KR beat Leigh to keep their hopes of winning the League Leaders Shield alive heading into the final week of the regular season.
Mikey Lewis scored two tries and four goals to set up an impressive 24-0 win at Leigh Sports Village that kept Rovers within touching distance of Super League leaders Wigan with a game to play.
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