British light heavyweight Joshua Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a tight 10-round bout at the Manchester venue.
But Parker had his moments, landing some telling clean blows in a match where the techniques failed to mesh, with excessive holding marring the action.
The officials had it 96-94 to the winner, with the final judge a even score.
"It wasn't my best performance but how do champions respond? They prevail," the victor said in the ringside as he was greeted with disapproval from the spectators.
The Londoner bounced back after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a 20th pro success.
Various analysts thought Parker, 31, merited the victory but the Derbyshire fighter experienced the second loss of his 28-fight career.
The Champion Claims Victory in Uneventful Encounter
The fighter's advancement has slowed in recent times through contract problems and long spells of inactivity.
Earlier forecast as the future of British boxing, he now stands trying to achieve expectations - perhaps in what feels like a final roll of the dice - under recent support.
He started carefully, with neither fighter prepared to be aggressive in a cagey initial rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second fight at the higher weight class.
By the halfway stage, the underdog had made it competitive, making life awkward for the favorite, who operated with a protective stance to barely take the more effective stanzas.
The spectators' acceptance wore thin because of constant clinching, with Parker going down multiple occasions during close exchanges.
Parker ended stronger, but many rounds were difficult to score. Nevertheless, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just a single round.
The victor advances, but with the likes of hard hitter Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels All-Manchester Fight
In the supporting bout, Arthur beat Bradley Rea by divided decision in a outstanding local rivalry contest to secure the European light-heavyweight belt.
The stadium fell silent for a extremely emotional tribute before the fight as ten bells rang in memory of the city's native Ricky Hatton, who passed away 46 years old in last month.
It was the first major televised boxing event in the urban center since Hatton's passing. As the last bell ended, the quiet became a roar of "there's only one Ricky Hatton".
The competition inside the ropes reflected the feeling. The eventual winner was knocked down by a powerful punch flush on the jaw in the early round, rising shakily to his feet on the eight count.
But the red-colored equipment of the fighter soon advanced again. From the other corner, the opponent - in opposing blue - was marked up early, heavy swelling developing under his eye as the fighters landed punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with stinging uppercuts as the action rarely dropped over twelve rounds.
Arthur edged the decision with judges scorecards of even score, close margin and wider gap, but the two athletes were relentless and courageous, everything their hero the legend embodied.
