UK 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 punch output likely gained him a split decision in a competitive ten-round contest at the Manchester venue.
But the challenger found success, landing some telling individual punches in a match where the approaches never quite clicked, with constant grabbing marring the action.
The officials had it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my greatest display but what defines champions? They win," the victor said in the ringside as he was greeted with disapproval from the crowd.
The Londoner got back to winning after his initial pro loss - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Various analysts felt Parker, thirty-one, deserved the win but the Derbyshire fighter suffered the second defeat of his 28-match professional journey.
The Champion Claims Victory in Uneventful Affair
Buatsi's progress has plateaued in recent times through promotional disputes and long spells of lack of fights.
Previously predicted as the next star of British boxing, he now stands trying to achieve expectations - perhaps in what appears to be a last chance - under recent support.
He began cautiously, with neither fighter prepared to take risks in a tentative early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after breaking his hand during the bout, was making just his second bout at the higher weight class.
By the middle rounds, the underdog had stayed in contention, creating difficulties for Buatsi, who worked behind a tight guard to edge the better stanzas.
The audience's tolerance decreased because of regular grabbing, with the challenger going down multiple occasions during clinches.
Parker finished the stronger, but multiple periods were hard to judge. Nevertheless, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor advances, but with the likes of power puncher David Benavidez, and Russian champion Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Hatton Memorial Fuels Local Rivalry Contest
In the secondary fight, Arthur beat Rea by split-decision in a outstanding all-Manchester battle to win the continental championship.
The venue became quiet for a profoundly moving tribute prior to the bout as a bell ceremony tolled in memory of local hero Ricky Hatton, who succumbed 46 years old in last month.
It was the first major televised boxing event in the city since Hatton's death. As the final chime ended, the quiet gave way to a shout of "there's just one Ricky Hatton".
The action within the ring matched the emotion. The eventual winner was knocked down by a powerful punch squarely on the face in the initial stage, getting up unsteadily to his position on the eight count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was marked up early, significant bruising developing under his face as the fighters exchanged hard punches.
Rea, 27, delivered effective strikes but the champion, 34, responded in kind with powerful upward punches as the action rarely dropped over 12 rounds.
Arthur barely secured the verdict with official scores of even score, close margin and wider gap, but both fighters were persistent and full of heart, all the qualities their idol the legend represented.
