British light-heavyweight Joshua Buatsi earned a win yet disappointing start to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a tight ten-round fight at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a contest where the approaches didn't complement each other, with frequent clinching marring the action.
The officials scored it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest performance but what do champions do? They win," Buatsi commented in the arena as he was faced boos from the spectators.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in February - to secure a 20th pro success.
Several boxing experts thought the challenger, 31, merited the decision but the contender endured the second defeat of his 28-match career.
The Champion Claims Win in Dull Match
Buatsi's progress has slowed in recent times through management issues and extended periods of inactivity.
Previously predicted as the next star of British boxing, he now stands trying to fulfil that early promise - possibly in what feels like a last chance - under fresh management.
He opened tentatively, with the two boxers prepared to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in two years ago after breaking his hand mid-fight, was making just his second fight at the higher weight class.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who utilized a protective stance to narrowly win the better rounds.
The audience's tolerance diminished amid frequent holding, with Parker slipping to the canvas several times during clinches.
Parker finished the stronger, but multiple periods were difficult to score. However, ex-champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian star Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Tribute Motivates Manchester Derby Fight
In the supporting bout, Arthur beat Bradley Rea by divided decision in a terrific local rivalry contest to secure the European light heavyweight championship.
The venue became quiet for a profoundly moving tribute prior to the bout as 10 bells sounded in memory of the city's native Hatton, who succumbed aged 46 in last month.
It was the first major televised boxing event in the location since Hatton's passing. As the final chime ended, the stillness transitioned to a shout of "there's only one Ricky Hatton".
The fighting within the ring reflected the feeling. Arthur was sent to the canvas by a significant blow directly to the face in the second round, standing uncertainly to his position on the eight count.
But the red-colored attire of the fighter soon came forward again. On the opposite side, the opponent - in competitive blue - was marked up quickly, significant bruising developing under his face as the pair landed hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with effective uppercuts as the intensity hardly diminished over 12 rounds.
Arthur narrowly won the result with official scores of draw, narrow lead and wider gap, but the two athletes were relentless and full of heart, exactly what their inspiration the legend embodied.
