UK light heavyweight Joshua Buatsi secured a victory yet unimpressive start to his career under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a tight 10-round contest at the boxing arena.
But Parker found success, landing some telling single shots in a match where the styles never quite clicked, with excessive holding spoiling the action.
Two judges had it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my greatest showing but how do champions respond? They prevail," the victor stated in the ringside as he was greeted with disapproval from the crowd.
The Londoner bounced back after his maiden career setback - an memorable fight with his previous opponent in last winter - to earn a 20th pro success.
Some ringside observers believed the challenger, thirty-one, merited the decision but the British boxer suffered the second loss of his 28-bout career.
Buatsi Claims Win in Slow Match
His career development has stalled in the past few years through contract problems and extended periods of inactivity.
Previously predicted as the future of British boxing, he now is positioned trying to achieve expectations - possibly in what feels like a ultimate opportunity - under recent support.
He started carefully, with the two boxers willing to be aggressive in a tentative opening few rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was participating in his second bout at the higher weight class.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who worked behind a tight guard to narrowly win the more effective periods.
The spectators' acceptance wore thin during regular grabbing, with the challenger falling to the floor several times during clinches.
Parker ended stronger, but many rounds were hard to judge. However, former world champion the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just a single round.
Buatsi moves on, but with the presence of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Memorial Fuels Manchester Derby Fight
In the supporting bout, Arthur overcame Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to secure the European light-heavyweight championship.
The stadium became quiet for a profoundly moving tribute ahead of the contest as a bell ceremony rang in remembrance of local hero Hatton, who succumbed 46 years old in recently.
It was the first major televised fight night in the location since Hatton's passing. As the last bell ended, the silence became a chant of "only one Hatton".
The fighting within the ring matched the emotion. Arthur was knocked down by a significant blow directly to the chin in the second round, getting up unsteadily to his footing on the eight count.
But the red-colored equipment of Arthur soon pressed ahead again. Across the ring, Rea - in competitive blue - was injured soon, significant bruising developing under his face as the fighters landed powerful shots.
Rea, 27, connected with strong blows but Arthur, 34, fought fire with fire with stinging uppercuts as the intensity hardly diminished over 12 rounds.
The victor edged the result with scoring tallies of 114-114, 115-113 and 115-112, but the two athletes were relentless and courageous, all the qualities their hero Hatton stood for.
