British light-heavyweight Joshua Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a tight 10-round bout at the Co-op Live Arena.
But Parker had his moments, delivering effective single shots in a match where the approaches never quite clicked, with frequent clinching spoiling the contest.
Two judges judged it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my best showing but what do champions do? They win," the victor commented in the ring as he was greeted with disapproval from the spectators.
The Londoner bounced back after his first professional defeat - an epic battle with his previous opponent in February - to secure a 20th career victory.
Several boxing experts felt the challenger, 31, merited the victory but the Derbyshire fighter endured the second setback of his 28-fight career.
Buatsi Secures Triumph in Slow Encounter
His career development has slowed in recent times through promotional disputes and extended periods of lack of fights.
Once tipped as the future of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers willing to take risks in a tentative early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand in the contest, was making just his second bout at light heavyweight.
By the halfway stage, the underdog had kept it close, making life awkward for Buatsi, who worked behind a solid defense to barely take the more effective stanzas.
The audience's tolerance decreased amid frequent holding, with Parker falling to the floor several times during clinches.
The underdog ended stronger, but multiple periods were hard to judge. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the likes of hard hitter Benavidez, and Russian star Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Hatton Memorial Motivates All-Manchester Fight
In the supporting bout, Lyndon Arthur defeated Rea by split-decision in a terrific Manchester vs Manchester contest to secure the European light-heavyweight championship.
The venue became quiet for a profoundly moving homage ahead of the contest as ten bells rang in memory of local hero Ricky Hatton, who passed away aged 46 in recently.
It was the first major televised boxing event in the location since the passing of Hatton. As the final chime faded, the stillness became a roar of "there's just one Ricky Hatton".
The fighting inside the ropes matched the feeling. The eventual winner was knocked down by a significant blow flush on the face in the initial stage, rising shakily to his feet on the referee's count.
But the red-colored shorts of Arthur soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was injured early, heavy swelling forming under his orbital area as the fighters landed punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with powerful upward punches as the action hardly diminished over the full distance.
The victor barely secured the verdict with judges scorecards of even score, 115-113 and 115-112, but both fighters were relentless and brave, exactly what their hero the legend represented.
