UK light heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a tight 10-round contest at the Co-op Live Arena.
But Parker found success, landing some telling clean blows in a fight where the approaches never quite clicked, with constant grabbing spoiling the action.
Two judges judged it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my greatest showing but what do champions do? They win," the victor said in the ringside as he was faced boos from the crowd.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with his previous opponent in last winter - to claim a twentieth professional win.
Various analysts believed Parker, thirty-one, should have been awarded the victory but the contender experienced the second loss of his 28-match professional journey.
Buatsi Earns Triumph in Dull Encounter
Buatsi's progress has slowed in recent times through management issues and extended periods of lack of fights.
Previously predicted as the future of the British fight scene, he now finds himself trying to realize his potential - maybe in what seems like a final roll of the dice - under recent support.
He opened tentatively, with the two boxers ready to take risks in a cagey initial rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in two years ago after damaging his hand during the bout, was competing in only his second fight at light heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who utilized a protective stance to barely take the better stanzas.
The audience's tolerance decreased during frequent holding, with Parker going down several times during clinches.
Parker closed well, but several stanzas were difficult to score. Nevertheless, previous titleholder Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
Buatsi advances, but with the presence of hard hitter David Benavidez, and Russian star Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Hatton Homage Inspires All-Manchester Fight
In the co-main event, Arthur overcame Rea by divided decision in a terrific all-Manchester contest to win the European light-heavyweight championship.
The arena fell silent for a deeply poignant memorial prior to the bout as 10 bells sounded in honor of the city's native Hatton, who died at age 46 in last month.
It was the premier televised boxing event in the urban center since Hatton's passing. As the last bell ended, the stillness became a shout of "only one Ricky Hatton".
The competition during the bout reflected the emotion. Arthur was knocked down by a powerful punch directly to the chin in the initial stage, standing uncertainly to his position on the eight count.
But the traditional red equipment of the fighter soon came forward again. On the opposite side, the opponent - in competitive blue - was damaged early, significant bruising forming under his orbital area as the fighters traded punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with stinging uppercuts as the intensity rarely dropped over 12 rounds.
Arthur narrowly won the decision with scoring tallies of even score, close margin and wider gap, but both fighters were determined and courageous, exactly what their idol the legend represented.
