The Rhinos, without linchpin Jake Connor, were void of ideas for most of the 80 minutes and never looked like clawing back an 8-0 half-time deficit.
The defeat means that Leeds have to rely on Leigh Leopards beating St Helens to ease fears that they will not qualify for a home semi-final.
And Arthur believes it is a chance gone begging.
“We’ve seen a couple of those performances this year and our losses look the same, we get into positions to win and find a way to lose. Every time we’ve chased a scoreboard, it hasn’t worked out,” Arthur said.
“We got a lot of big plays wrong, and I don’t know where it’s come from, it’s not a reflection of us.
“We tried too hard, needed to calm down and get back into the cycle. Not having Jake should be an excuse.
“We just 17 needed blokes to do their job and I’m not taking any credit away from Catalans, they fought really hard to stop us but it led to our frustration.
“We’ve responded before, we know what works for us, and that’s all that matters, it’s a pretty easy fix.
“We’ve let an opportunity go and that’s disappointing.”