UK light-heavyweight Joshua Buatsi secured a victory yet disappointing beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a judges' verdict in a close 10-round contest at the Co-op Live Arena.
But the challenger found success, connecting with powerful clean blows in a fight where the techniques didn't complement each other, with excessive holding disrupting the fight.
The officials had it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my greatest display but what defines champions? They succeed," Buatsi commented in the arena as he was faced boos from the crowd.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to secure a twentieth professional win.
Several boxing experts thought the challenger, 31, deserved the win but the contender experienced the second loss of his 28-match career.
Buatsi Claims Victory in Uneventful Affair
The fighter's advancement has stalled in recent years through management issues and considerable time of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to realize his potential - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter prepared to take risks in a calculated initial rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was competing in only his second bout at the higher weight class.
By the halfway stage, Parker had kept it close, causing problems for the favorite, who worked behind a protective stance to barely take the better periods.
The crowd's patience decreased because of frequent holding, with Parker going down various instances during grappling.
Parker finished the stronger, but multiple periods were difficult to score. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just one round.
Buatsi moves on, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still seems difficult.
Hatton Memorial Motivates All-Manchester Contest
In the supporting bout, Arthur overcame Bradley Rea by split-decision in a outstanding all-Manchester fight to secure the European light heavyweight title.
The arena became quiet for a extremely emotional tribute before the fight as ten bells tolled in honor of local hero Hatton, who succumbed at age 46 in September.
It was the premier televised fight night in the urban center since Hatton's passing. As the last bell faded, the quiet became a chant of "there's just one Ricky Hatton".
The action inside the ropes matched the emotion. The eventual winner was floored by a significant blow flush on the jaw in the second round, standing uncertainly to his footing on the eight count.
But the Manchester United-red attire of Arthur soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was damaged early, noticeable damage forming under his eye as the duo traded hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with stinging uppercuts as the pace rarely dropped over twelve rounds.
The victor edged the result with official scores of even score, close margin and 115-112, but both fighters were determined and courageous, exactly what their hero the legend represented.
