UK light-heavyweight Joshua Buatsi made a winning yet underwhelming debut to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a split decision in a close 10-round contest at the boxing arena.
But Parker had his moments, delivering effective individual punches in a match where the techniques failed to mesh, with frequent clinching disrupting the action.
The scoring judges scored it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my best performance but what defines champions? They win," Buatsi commented in the arena as he was greeted with disapproval from the spectators.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in last winter - to earn a 20th pro success.
Some ringside observers thought Parker, thirty-one, merited the win but the British boxer suffered the second setback of his 28-fight career.
The Champion Claims Triumph in Uneventful Encounter
His career development has plateaued in recent years through promotional disputes and long spells of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what feels like a last chance - under fresh management.
He began cautiously, with the two boxers ready to be aggressive in a cagey initial rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in two years ago after suffering a hand injury mid-fight, was competing in only his second bout at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who utilized a tight guard to narrowly win the cleaner stanzas.
The spectators' acceptance decreased amid frequent holding, with the challenger slipping to the canvas several times during clinches.
Parker ended stronger, but several stanzas were difficult to score. Nevertheless, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just one round.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Hatton Homage Motivates All-Manchester Battle
In the co-main event, Arthur beat Bradley Rea by split-decision in a outstanding local rivalry battle to capture the continental championship.
The arena fell silent for a profoundly moving tribute ahead of the contest as ten bells sounded in memory of Manchester's own Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the concluding sound faded, the silence gave way to a roar of "there's just one Hatton".
The fighting during the bout reflected the sentiment. Arthur was sent to the canvas by a powerful punch squarely on the face in the early round, standing uncertainly to his footing on the referee's count.
But the Manchester United-red shorts of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was marked up soon, heavy swelling forming under his face as the fighters landed punishing blows.
Rea, 27, landed power punches but Arthur, 34, responded in kind with powerful upward punches as the action seldom decreased over twelve rounds.
The victor barely secured the result with judges scorecards of draw, 115-113 and clearer margin, but each competitor were persistent and brave, everything their idol Hatton represented.
