UK light heavyweight Joshua Buatsi made a winning yet disappointing start to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a tight ten-round bout at the Co-op Live Arena.
But Parker had his moments, connecting with powerful single shots in a contest where the approaches failed to mesh, with constant grabbing spoiling the action.
The officials scored it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my finest showing but what do champions do? They win," Buatsi said in the arena as he was greeted with negative reactions from the spectators.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a 20th pro success.
Some ringside observers felt Parker, thirty-one, deserved the decision but the British boxer suffered the second defeat of his 28-fight career.
The Champion Secures Victory in Dull Match
Buatsi's progress has slowed in recent times through management issues and considerable time of inactivity.
Earlier forecast as the rising talent of British boxing, he now stands trying to realize his potential - perhaps in what feels like a ultimate opportunity - under fresh management.
He started carefully, with both competitors prepared to be aggressive in a tentative opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in two years ago after breaking his hand mid-fight, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who worked behind a tight guard to narrowly win the cleaner rounds.
The crowd's patience diminished amid frequent holding, with Parker slipping to the canvas multiple occasions during close exchanges.
Parker ended stronger, but many rounds were difficult to score. Although, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
The victor advances, but with the presence of power puncher David Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Hatton Memorial Motivates All-Manchester Battle
In the secondary fight, Arthur overcame Bradley Rea by split-decision in a excellent Manchester vs Manchester fight to capture the European light heavyweight belt.
The arena grew hushed for a deeply poignant memorial prior to the bout as ten bells sounded in memory of Manchester's own Ricky Hatton, who passed away aged 46 in last month.
It was the premier televised fight night in the city since the passing of Hatton. As the last bell ended, the quiet transitioned to a chant of "there's just one Hatton".
The action during the bout equaled the emotion. The eventual winner was floored by a significant blow squarely on the face in the early round, standing uncertainly to his footing on the referee's count.
But the traditional red shorts of Arthur soon came forward again. From the other corner, the opponent - in rival City blue - was marked up soon, significant bruising appearing under his face as the fighters landed punishing blows.
Rea, 27, connected with strong blows but the champion, 34, fought fire with fire with stinging uppercuts as the action hardly diminished over twelve rounds.
The victor barely secured the verdict with judges scorecards of even score, 115-113 and wider gap, but the two athletes were persistent and full of heart, everything their hero Hatton stood for.
