British light-heavyweight Joshua Buatsi earned a win yet disappointing start to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a tight ten-round bout at the Co-op Live Arena.
But the challenger had his moments, landing some telling individual punches in a contest where the approaches didn't complement each other, with frequent clinching spoiling the action.
Two judges judged it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my greatest display but what do champions do? They succeed," the victor said in the arena as he was faced negative reactions from the spectators.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a 20th pro success.
Several boxing experts thought Parker, thirty-one, deserved the decision but the contender endured the second setback of his 28-fight professional journey.
Buatsi Secures Triumph in Slow Affair
His career development has plateaued in recent times through management issues and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to achieve expectations - maybe in what feels like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors willing to overcommit in a calculated initial rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in two years ago after breaking his hand in the contest, was participating in his second fight at light heavyweight.
By the halfway stage, the underdog had kept it close, creating difficulties for the favorite, who utilized a protective stance to narrowly win the cleaner rounds.
The spectators' acceptance diminished because of constant clinching, with Parker falling to the floor various instances during grappling.
Parker finished the stronger, but many rounds were challenging to assess. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Memorial Inspires Local Rivalry Battle
In the secondary fight, Arthur beat Rea by close verdict in a outstanding Manchester vs Manchester contest to win the European light heavyweight title.
The stadium became quiet for a profoundly moving memorial prior to the bout as ten bells rang in honor of local hero Hatton, who passed away 46 years old in September.
It was the premier televised boxing event in the urban center since Hatton's death. As the final chime ended, the stillness gave way to a shout of "only one Hatton".
The fighting within the ring matched the emotion. The eventual winner was knocked down by a left hook directly to the chin in the initial stage, standing uncertainly to his footing on the eight count.
But the traditional red attire of Arthur soon pressed ahead again. Across the ring, Rea - in rival City blue - was marked up quickly, significant bruising forming under his face as the pair exchanged punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, 34, fought fire with fire with effective uppercuts as the intensity hardly diminished over 12 rounds.
The victor barely secured the verdict with scoring tallies of even score, narrow lead and clearer margin, but each competitor were determined and courageous, exactly what their idol the legend represented.
