British light-heavyweight Joshua Buatsi made a winning yet unimpressive start to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a majority decision in a close 10-round fight at the boxing arena.
But Parker had his moments, connecting with powerful individual punches in a fight where the styles failed to mesh, with excessive holding spoiling the action.
Two judges judged it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my best display but how do champions respond? They succeed," Buatsi stated in the ring as he was met by boos from the crowd.
The Londoner bounced back after his initial pro loss - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers thought the challenger, thirty-one, should have been awarded the victory but the contender endured the second defeat of his 28-bout professional journey.
The Champion Secures Triumph in Slow Match
His career development has stalled in recent years through contract problems and long spells of inactivity.
Previously predicted as the next star of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under fresh management.
He opened tentatively, with both competitors willing to overcommit in a tentative early stages.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was participating in his second appearance at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for the favorite, who operated with a protective stance to barely take the cleaner periods.
The crowd's patience diminished amid frequent holding, with the challenger going down several times during grappling.
Parker ended stronger, but many rounds were challenging to assess. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi progresses, but with the presence of power puncher Benavidez, and Russian champion Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the secondary fight, Arthur beat Rea by split-decision in a terrific Manchester vs Manchester contest to secure the European light heavyweight belt.
The venue became quiet for a deeply poignant tribute prior to the bout as ten bells tolled in honor of the city's native Ricky Hatton, who died aged 46 in recently.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the final chime faded, the silence transitioned to a roar of "only one Ricky Hatton".
The action within the ring reflected the sentiment. The eventual winner was knocked down by a left hook squarely on the chin in the second round, getting up unsteadily to his footing on the eight count.
But the Manchester United-red shorts of Arthur soon advanced again. Across the ring, Rea - in competitive blue - was damaged early, noticeable damage appearing under his eye as the duo traded punishing blows.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the action hardly diminished over twelve rounds.
The victor narrowly won the result with judges scorecards of 114-114, narrow lead and clearer margin, but the two athletes were determined and courageous, everything their idol the legend stood for.
