British light heavyweight Joshua Buatsi earned a win yet unimpressive start to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a tight 10-round bout at the Co-op Live Arena.
But Parker found success, connecting with powerful single shots in a contest where the approaches never quite clicked, with constant grabbing marring the contest.
Two judges had it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my best showing but how do champions respond? They win," Buatsi commented in the ring as he was met by disapproval from the spectators.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in last winter - to secure a 20th career victory.
Several boxing experts believed Parker, 31, deserved the win but the British boxer experienced the second loss of his 28-bout career.
The Champion Earns Win in Dull Encounter
Buatsi's progress has slowed in recent times through promotional disputes and considerable time of inactivity.
Earlier forecast as the future of British boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a last chance - under new promotional backing.
He began cautiously, with the two boxers willing to overcommit in a cagey early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after suffering a hand injury during the bout, was participating in his second fight at light heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for Buatsi, who operated with a protective stance to narrowly win the better stanzas.
The crowd's patience diminished amid regular grabbing, with the challenger slipping to the canvas several times during grappling.
Parker closed well, but multiple periods were hard to judge. However, ex-champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Tribute Motivates All-Manchester Fight
In the secondary fight, Arthur beat Bradley Rea by close verdict in a excellent local rivalry contest to secure the European light-heavyweight belt.
The stadium grew hushed for a deeply poignant tribute prior to the bout as 10 bells sounded in remembrance of Manchester's own Ricky Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast match card in the location since the passing of Hatton. As the final chime disappeared, the stillness transitioned to a chant of "only one Hatton".
The action within the ring matched the sentiment. Arthur was sent to the canvas by a left hook directly to the face in the second round, rising shakily to his footing on the referee's count.
But the traditional red attire of the fighter soon came forward again. Across the ring, the opponent - in rival City blue - was marked up early, significant bruising appearing under his face as the duo traded punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, 34, matched the intensity with powerful upward punches as the intensity rarely dropped over the full distance.
The victor narrowly won the result with official scores of draw, 115-113 and 115-112, but each competitor were determined and brave, all the qualities their idol Hatton embodied.
