British light-heavyweight Joshua Buatsi made a winning yet disappointing beginning to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a majority decision in a close 10-round contest at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling individual punches in a fight where the styles never quite clicked, with frequent clinching disrupting the fight.
The scoring judges scored it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my finest display but how do champions respond? They succeed," the victor said in the ring as he was greeted with negative reactions from the crowd.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in February - to secure a 20th career victory.
Some ringside observers believed Parker, 31, merited the win but the Derbyshire fighter suffered the second setback of his 28-fight professional journey.
The Champion Earns Triumph in Uneventful Encounter
The fighter's advancement has slowed in the past few years through management issues and long spells of lack of fights.
Previously predicted as the future of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with the two boxers willing to be aggressive in a cagey opening few rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was participating in his second fight at light-heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for Buatsi, who worked behind a tight guard to barely take the more effective stanzas.
The audience's tolerance decreased amid regular grabbing, with Parker falling to the floor several times during grappling.
The underdog finished the stronger, but multiple periods were difficult to score. Although, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the presence of power puncher David Benavidez, and Russian star Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Memorial Fuels All-Manchester Fight
In the co-main event, Arthur defeated Bradley Rea by close verdict in a terrific local rivalry fight to win the European light-heavyweight belt.
The stadium fell silent for a profoundly moving homage before the fight as ten bells rang in honor of local hero Ricky Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast fight night in the location since the passing of Hatton. As the concluding sound faded, the quiet became a roar of "only one Ricky Hatton".
The competition inside the ropes equaled the sentiment. Arthur was sent to the canvas by a left hook flush on the chin in the initial stage, rising shakily to his feet on the eight count.
But the traditional red attire of Arthur soon came forward again. Across the ring, Rea - in competitive blue - was damaged soon, noticeable damage forming under his face as the pair traded punishing blows.
Rea, 27, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the action rarely dropped over twelve rounds.
The victor edged the result with judges scorecards of draw, narrow lead and wider gap, but both fighters were persistent and full of heart, exactly what their inspiration Hatton stood for.
