UK light-heavyweight Joshua Buatsi made a winning yet underwhelming debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a tight 10-round fight at the Manchester venue.
But the challenger had his moments, landing some telling individual punches in a contest where the approaches failed to mesh, with frequent clinching disrupting the action.
Two judges scored it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest display but what defines champions? They succeed," the victor said in the ringside as he was greeted with negative reactions from the audience.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to earn a 20th career victory.
Some ringside observers thought the challenger, 31, merited the win but the Derbyshire fighter endured the second loss of his 28-match professional journey.
Buatsi Secures Win in Uneventful Match
His career development has stalled in recent times through promotional disputes and long spells of inactivity.
Earlier forecast as the rising talent of British boxing, he now stands trying to fulfil that early promise - possibly in what seems like a last chance - under fresh management.
He began cautiously, with both competitors ready to be aggressive in a cagey early stages.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was making just his second fight at light heavyweight.
By the middle rounds, the underdog had kept it close, making life awkward for Buatsi, who worked behind a tight guard to barely take the better periods.
The crowd's patience decreased during frequent holding, with Parker falling to the floor several times during grappling.
Parker closed well, but multiple periods were challenging to assess. However, ex-champion Carl Frampton - working with Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels Manchester Derby Fight
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a outstanding all-Manchester battle to win the continental title.
The arena became quiet for a deeply poignant memorial before the fight as ten bells sounded in honor of local hero Ricky Hatton, who died at age 46 in September.
It was the first major televised boxing event in the city since the passing of Hatton. As the concluding sound ended, the quiet gave way to a shout of "there's only one Hatton".
The competition during the bout matched the feeling. The eventual winner was floored by a powerful punch squarely on the jaw in the second round, getting up unsteadily to his position on the eight count.
But the red-colored shorts of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was marked up quickly, noticeable damage developing under his eye as the fighters landed punishing blows.
Rea, twenty-seven, landed power punches but Arthur, 34, responded in kind with powerful upward punches as the pace hardly diminished over the full distance.
The victor edged the result with judges scorecards of draw, close margin and clearer margin, but both fighters were determined and courageous, exactly what their hero the legend stood for.
