UK light heavyweight Joshua Buatsi secured a victory yet disappointing start to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a split decision in a tight ten-round fight at the Manchester venue.
But the challenger landed significant shots, landing some telling clean blows in a match where the approaches didn't complement each other, with constant grabbing disrupting the contest.
The officials scored it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest performance but what defines champions? They prevail," the victor stated in the arena as he was greeted with negative reactions from the audience.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Several boxing experts believed Parker, thirty-one, should have been awarded the victory but the British boxer endured the second defeat of his 28-match professional journey.
Buatsi Claims Triumph in Slow Encounter
His career development has plateaued in recent times through management issues and long spells of inactivity.
Once tipped as the future of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what appears to be a last chance - under new promotional backing.
He opened tentatively, with neither fighter prepared to overcommit in a cagey opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in two years ago after breaking his hand in the contest, was making just his second appearance at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, making life awkward for Buatsi, who operated with a protective stance to narrowly win the cleaner rounds.
The spectators' acceptance decreased because of constant clinching, with the challenger going down various instances during grappling.
The underdog closed well, but many rounds were difficult to score. However, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the likes of hard hitter David Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - based on this performance - still looks steep.
Hatton Homage Fuels Local Rivalry Battle
In the co-main event, Arthur defeated Rea by split-decision in a excellent all-Manchester fight to secure the continental championship.
The stadium grew hushed for a deeply poignant homage prior to the bout as ten bells sounded in honor of the city's native Ricky Hatton, who passed away aged 46 in recently.
It was the premier televised boxing event in the city since the passing of Hatton. As the concluding sound faded, the stillness gave way to a roar of "there's only one Hatton".
The action within the ring reflected the feeling. Arthur was sent to the canvas by a significant blow directly to the face in the early round, rising shakily to his position on the count of eight.
But the Manchester United-red shorts of Arthur soon pressed ahead again. From the other corner, Rea - in opposing blue - was damaged quickly, heavy swelling appearing under his eye as the pair traded punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the intensity hardly diminished over twelve rounds.
Arthur edged the verdict with official scores of even score, narrow lead and clearer margin, but the two athletes were relentless and full of heart, everything their inspiration Hatton stood for.
