UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a tight 10-round fight at the Co-op Live Arena.
But Parker found success, delivering effective clean blows in a match where the techniques never quite clicked, with excessive holding disrupting the contest.
The officials judged it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my best performance but how do champions respond? They succeed," the victor stated in the ring as he was faced boos from the audience.
The British fighter got back to winning after his first professional defeat - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Several boxing experts thought Parker, thirty-one, merited the victory but the contender suffered the second loss of his 28-match professional journey.
The Champion Claims Win in Dull Encounter
His career development has slowed in the past few years through promotional disputes and long spells of inactivity.
Once tipped as the future of UK boxing, he now is positioned trying to realize his potential - perhaps in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with neither fighter willing to take risks in a calculated initial rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after damaging his hand during the bout, was competing in only his second fight at light-heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who utilized a tight guard to barely take the more effective rounds.
The spectators' acceptance decreased amid regular grabbing, with the challenger falling to the floor several times during clinches.
Parker ended stronger, but several stanzas were hard to judge. However, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - based on this performance - still appears challenging.
Hatton Tribute Fuels All-Manchester Contest
In the supporting bout, Arthur overcame Rea by split-decision in a outstanding Manchester vs Manchester battle to secure the continental belt.
The stadium grew hushed for a profoundly moving tribute before the fight as 10 bells sounded in honor of Manchester's own Ricky Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the concluding sound ended, the quiet became a shout of "only one Ricky Hatton".
The fighting during the bout matched the sentiment. The eventual winner was knocked down by a left hook flush on the chin in the initial stage, rising shakily to his position on the referee's count.
But the traditional red equipment of the fighter soon pressed ahead again. Across the ring, the opponent - in rival City blue - was injured soon, noticeable damage forming under his orbital area as the pair exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the pace rarely dropped over the full distance.
Arthur narrowly won the verdict with official scores of draw, close margin and clearer margin, but each competitor were relentless and brave, everything their idol Hatton embodied.
