British light-heavyweight Joshua Buatsi earned a win yet underwhelming beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a majority decision in a competitive ten-round contest at the Co-op Live Arena.
But the challenger found success, connecting with powerful clean blows in a match where the styles never quite clicked, with excessive holding spoiling the action.
Two judges had it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my best display but how do champions respond? They prevail," the victor stated in the arena as he was met by disapproval from the spectators.
The Londoner bounced back after his maiden career setback - an memorable fight with Callum Smith in February - to earn a 20th career victory.
Some ringside observers thought Parker, thirty-one, deserved the decision but the British boxer endured the second setback of his 28-fight professional journey.
Buatsi Earns Victory in Slow Affair
Buatsi's progress has stalled in recent times through contract problems and considerable time of inactivity.
Previously predicted as the future of British boxing, he now stands trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors ready to overcommit in a tentative opening few rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after breaking his hand during the bout, was competing in only his second appearance at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for Buatsi, who operated with a solid defense to narrowly win the better rounds.
The crowd's patience diminished amid constant clinching, with Parker slipping to the canvas several times during clinches.
Parker ended stronger, but multiple periods were challenging to assess. Nevertheless, former world champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
The victor moves on, but with the likes of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Tribute Fuels All-Manchester Contest
In the secondary fight, Arthur defeated Bradley Rea by split-decision in a outstanding all-Manchester battle to secure the continental championship.
The stadium became quiet for a deeply poignant tribute before the fight as 10 bells sounded in remembrance of local hero Hatton, who died 46 years old in recently.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the final chime faded, the stillness became a shout of "only one Ricky Hatton".
The action within the ring matched the feeling. Arthur was knocked down by a significant blow directly to the chin in the second round, standing uncertainly to his feet on the referee's count.
But the Manchester United-red attire of the fighter soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was injured soon, heavy swelling forming under his face as the duo exchanged hard punches.
Rea, 27, connected with strong blows but the champion, 34, fought fire with fire with stinging uppercuts as the intensity hardly diminished over 12 rounds.
Arthur narrowly won the decision with official scores of 114-114, 115-113 and clearer margin, but both fighters were persistent and courageous, all the qualities their idol the legend stood for.
