British light heavyweight Joshua Buatsi secured a victory yet unimpressive debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a tight 10-round contest at the boxing arena.
But Parker landed significant shots, connecting with powerful single shots in a fight where the techniques didn't complement each other, with frequent clinching spoiling the contest.
The officials had it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest display but how do champions respond? They prevail," the victor commented in the ringside as he was greeted with disapproval from the crowd.
The Londoner got back to winning after his first professional defeat - an epic battle with his previous opponent in February - to earn a 20th pro success.
Some ringside observers believed Parker, 31, deserved the win but the British boxer endured the second loss of his 28-fight professional journey.
The Champion Claims Win in Dull Encounter
His career development has stalled in recent years through contract problems and considerable time of inactivity.
Earlier forecast as the future of UK boxing, he now finds himself trying to fulfil that early promise - maybe in what seems like a ultimate opportunity - under recent support.
He began cautiously, with the two boxers willing to take risks in a calculated opening few rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was making just his second bout at light heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for Buatsi, who worked behind a solid defense to barely take the better periods.
The audience's tolerance decreased during regular grabbing, with the challenger slipping to the canvas various instances during clinches.
Parker ended stronger, but many rounds were hard to judge. Although, ex-champion the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the caliber of power puncher David Benavidez, and Russian champion Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Battle
In the supporting bout, Arthur defeated Bradley Rea by split-decision in a outstanding Manchester vs Manchester contest to capture the continental title.
The venue became quiet for a extremely emotional tribute before the fight as a bell ceremony tolled in honor of local hero Ricky Hatton, who succumbed at age 46 in recently.
It was the premier televised match card in the city since Hatton's death. As the concluding sound faded, the stillness gave way to a shout of "only one Ricky Hatton".
The competition within the ring reflected the sentiment. The eventual winner was knocked down by a powerful punch flush on the face in the second round, rising shakily to his feet on the eight count.
But the red-colored shorts of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was injured soon, noticeable damage appearing under his orbital area as the fighters exchanged powerful shots.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with powerful upward punches as the pace hardly diminished over the full distance.
Arthur barely secured the result with judges scorecards of even score, close margin and wider gap, but the two athletes were determined and courageous, everything their inspiration the legend embodied.
