UK light-heavyweight Joshua Buatsi earned a win yet disappointing debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a majority decision in a tight ten-round fight at the boxing arena.
But Parker found success, landing some telling single shots in a fight where the styles never quite clicked, with frequent clinching spoiling the action.
The scoring judges had it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my best performance but what do champions do? They prevail," Buatsi commented in the arena as he was greeted with boos from the audience.
The British fighter got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to earn a 20th pro success.
Some ringside observers felt Parker, 31, merited the win but the contender endured the second defeat of his 28-fight career.
The Champion Earns Win in Slow Affair
The fighter's advancement has plateaued in recent times through promotional disputes and long spells of lack of fights.
Earlier forecast as the future of British boxing, he now stands trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter ready to take risks in a tentative early stages.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was participating in his second bout at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who utilized a protective stance to edge the better rounds.
The audience's tolerance decreased amid frequent holding, with the challenger falling to the floor multiple occasions during grappling.
The underdog closed well, but many rounds were hard to judge. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi progresses, but with the presence of hard hitter Benavidez, and Russian star Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Hatton Tribute Fuels Manchester Derby Battle
In the secondary fight, Arthur overcame Bradley Rea by close verdict in a outstanding local rivalry fight to win the European light-heavyweight belt.
The arena grew hushed for a profoundly moving tribute prior to the bout as ten bells sounded in honor of Manchester's own Hatton, who succumbed at age 46 in recently.
It was the premier televised boxing event in the city since the passing of Hatton. As the final chime ended, the quiet gave way to a shout of "there's just one Hatton".
The fighting during the bout equaled the emotion. The eventual winner was floored by a left hook squarely on the jaw in the early round, getting up unsteadily to his position on the eight count.
But the red-colored shorts of the fighter soon pressed ahead again. On the opposite side, Rea - in competitive blue - was damaged soon, noticeable damage appearing under his face as the fighters traded hard punches.
The challenger, 27, landed power punches but the champion, 34, responded in kind with effective uppercuts as the action hardly diminished over 12 rounds.
Arthur narrowly won the decision with judges scorecards of even score, narrow lead and wider gap, but each competitor were relentless and full of heart, everything their hero the legend embodied.
