British light heavyweight Joshua Buatsi secured a victory yet underwhelming start to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a tight ten-round fight at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective single shots in a match where the styles failed to mesh, with frequent clinching marring the contest.
Two judges had it 96-94 for to the winner, with the remaining official a 95-95.
"It wasn't my greatest showing but what defines champions? They win," Buatsi stated in the arena as he was greeted with negative reactions from the crowd.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts felt Parker, thirty-one, deserved the win but the Derbyshire fighter suffered the second loss of his 28-bout career.
Buatsi Secures Victory in Slow Match
Buatsi's progress has plateaued in the past few years through promotional disputes and long spells of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - possibly in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter prepared to overcommit in a calculated early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in two years ago after suffering a hand injury during the bout, was participating in his second bout at light-heavyweight.
By the middle rounds, Parker had kept it close, causing problems for Buatsi, who worked behind a tight guard to narrowly win the better rounds.
The audience's tolerance wore thin during constant clinching, with Parker going down multiple occasions during clinches.
The underdog ended stronger, but multiple periods were hard to judge. Although, ex-champion the boxing analyst - working with the broadcast network - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi moves on, but with the caliber of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Hatton Tribute Inspires Manchester Derby Contest
In the supporting bout, Arthur overcame Rea by divided decision in a excellent local rivalry contest to secure the continental title.
The stadium grew hushed for a extremely emotional homage before the fight as 10 bells tolled in memory of the city's native Ricky Hatton, who died aged 46 in September.
It was the first major televised match card in the city since Hatton's passing. As the final chime disappeared, the stillness transitioned to a roar of "there's just one Hatton".
The competition within the ring reflected the feeling. The eventual winner was knocked down by a powerful punch flush on the chin in the initial stage, rising shakily to his feet on the eight count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. From the other corner, the opponent - in opposing blue - was damaged early, heavy swelling appearing under his eye as the pair traded hard punches.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with powerful upward punches as the action hardly diminished over 12 rounds.
Arthur edged the decision with scoring tallies of draw, close margin and 115-112, but each competitor were relentless and courageous, exactly what their inspiration Hatton stood for.
