British light-heavyweight Joshua Buatsi earned a win yet disappointing beginning to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a competitive ten-round contest at the Co-op Live Arena.
But Parker had his moments, landing some telling clean blows in a fight where the techniques never quite clicked, with excessive holding spoiling the fight.
The scoring judges had it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my best showing but what defines champions? They win," Buatsi said in the ring as he was greeted with negative reactions from the crowd.
The Londoner bounced back after his maiden career setback - an epic battle with his previous opponent in February - to claim a twentieth professional win.
Various analysts thought the challenger, 31, should have been awarded the victory but the British boxer experienced the second setback of his 28-fight career.
The Champion Claims Triumph in Slow Affair
Buatsi's progress has slowed in the past few years through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of UK boxing, he now is positioned trying to realize his potential - possibly in what feels like a last chance - under new promotional backing.
He started carefully, with the two boxers willing to be aggressive in a tentative early stages.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand during the bout, was participating in his second appearance at the higher weight class.
By the fight's midpoint, the underdog had kept it close, causing problems for the favorite, who operated with a solid defense to barely take the cleaner rounds.
The spectators' acceptance wore thin because of frequent holding, with the challenger going down various instances during grappling.
The underdog closed well, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, awarding Buatsi just one round.
The victor progresses, but with the caliber of hard hitter Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Homage Inspires All-Manchester Fight
In the co-main event, Arthur overcame Rea by split-decision in a terrific Manchester vs Manchester fight to capture the continental championship.
The stadium grew hushed for a profoundly moving homage prior to the bout as 10 bells sounded in remembrance of the city's native Hatton, who succumbed 46 years old in last month.
It was the premier televised fight night in the location since Hatton's passing. As the last bell disappeared, the quiet became a roar of "there's only one Hatton".
The competition during the bout matched the feeling. The eventual winner was sent to the canvas by a powerful punch flush on the chin in the initial stage, rising shakily to his position on the referee's count.
But the red-colored equipment of the fighter soon came forward again. On the opposite side, Rea - in competitive blue - was damaged soon, heavy swelling developing under his face as the fighters exchanged hard punches.
Rea, 27, connected with strong blows but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity seldom decreased over 12 rounds.
The victor barely secured the result with scoring tallies of draw, close margin and clearer margin, but each competitor were persistent and brave, all the qualities their inspiration Hatton stood for.
