British light heavyweight Joshua Buatsi made a winning yet underwhelming beginning to his career under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a close ten-round bout at the boxing arena.
But Parker found success, landing some telling clean blows in a match where the techniques didn't complement each other, with frequent clinching disrupting the action.
The scoring judges scored it 96-94 to the winner, with the third a 95-95.
"It wasn't my best performance but what do champions do? They succeed," the victor said in the arena as he was greeted with boos from the audience.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a 20th career victory.
Some ringside observers believed the challenger, 31, merited the decision but the British boxer suffered the second defeat of his 28-fight professional journey.
Buatsi's progress has plateaued in recent years through contract problems and extended periods of inactivity.
Once tipped as the future of UK boxing, he now stands trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors ready to overcommit in a tentative early stages.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after damaging his hand in the contest, was participating in his second appearance at light-heavyweight.
By the halfway stage, Parker had stayed in contention, causing problems for Buatsi, who operated with a tight guard to barely take the better stanzas.
The crowd's patience wore thin because of frequent holding, with the challenger going down various instances during clinches.
Parker finished the stronger, but several stanzas were difficult to score. However, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
In the co-main event, Arthur overcame Rea by close verdict in a terrific all-Manchester contest to capture the European light-heavyweight title.
The venue fell silent for a profoundly moving homage ahead of the contest as ten bells tolled in memory of local hero Ricky Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast match card in the city since Hatton's death. As the last bell faded, the quiet became a chant of "only one Hatton".
The action during the bout equaled the feeling. The eventual winner was sent to the canvas by a significant blow squarely on the jaw in the initial stage, getting up unsteadily to his position on the eight count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was damaged quickly, noticeable damage appearing under his face as the duo landed hard punches.
Rea, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the intensity hardly diminished over twelve rounds.
Arthur edged the result with scoring tallies of 114-114, narrow lead and clearer margin, but both fighters were persistent and full of heart, all the qualities their inspiration the legend stood for.
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