The Sea Eagles have confirmed Arthur's immediate signing and will make him available for Sunday's clash against Newcastle, which Daly Cherry-Evans is missing through State of Origin duty.
Arthur joins the top-30 squad on Tuesday and will bolster a Sea Eagles side struggling for stability in the halves, with a string of injuries scuppering new five-eighth Josh Schuster this season.
"We feel he also has got the ability to be a ball-playing forward with his size and defensive capabilities,'' Manly coach Anthony Seibold said.
The son of Eels coach Brad, Arthur had been a fringe first-grader with the Eels since debuting at Magic Round in 2021, mostly earning selection when Moses or five-eighth Dylan Brown were unavailable.
He notably filled in when a finger injury sidelined Moses just as Parramatta were vying to qualify for the top-four last year, and later secured a spot on the bench on the run to the grand final.
"Jake is a massive loss for us," Moses said.
"Anytime we were out, he filled in and did his job. I can't fault him for that.
"He was playing really good footy in reserve grade so happy for him to get a deal somewhere else. He'll be a really big asset for wherever he goes.
"We're going to miss him here a lot."
Arthur's time at the Eels was marred by negative treatment from Parramatta fans.
Early last season, the Eels came out in defence of Arthur, who received social media trolling for being named at five-eighth ahead of Brown in two games.
Later in the season, sections of the Parramatta home crowd booed the young playmaker, with reports emerging he had been targeted by Eels fans after his lone first-grade appearance this year.
As Arthur prepares to leave, Moses reiterated the 20-year-old had never been picked through nepotism, and that he never deserved such treatment.
"It's upsetting to see (the booing) because he's such a great kid and puts so much time into his game," he said.
"To see people go at him in that way, and it's your own fans, it's pretty sad.
"He deserved to be there. It's not that Brad's the coach and that's why he's getting picked. I think the games that he did fill in for us, he did his job.
"It was probably our senior boys that needed to help him a bit more, really, and put the pressure on us. Put the blame on us, not him."
Moses had verbally committed to an Eels contract extension earlier in the season but officially confirmed his new deal last week.
"I grew up supporting the club so it was a bit hard for me to leave," he said.
"I'm glad it's all sorted."
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