UK light heavyweight Joshua Buatsi earned a win yet underwhelming debut to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a competitive ten-round bout at the Co-op Live Arena.
But the challenger found success, delivering effective single shots in a contest where the styles failed to mesh, with constant grabbing marring the fight.
Two judges had it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my finest showing but what defines champions? They win," the victor commented in the ringside as he was greeted with disapproval from the crowd.
The Londoner returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to earn a 20th career victory.
Several boxing experts thought Parker, 31, merited the decision but the contender endured the second setback of his 28-fight career.
The Champion Claims Victory in Dull Affair
Buatsi's progress has slowed in recent years through promotional disputes and considerable time of lack of fights.
Previously predicted as the future of UK boxing, he now finds himself trying to realize his potential - possibly in what appears to be a last chance - under new promotional backing.
He began cautiously, with both competitors prepared to be aggressive in a cagey early stages.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second appearance at the higher weight class.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who utilized a protective stance to barely take the better periods.
The spectators' acceptance decreased during constant clinching, with the challenger going down multiple occasions during grappling.
The underdog closed well, but multiple periods were challenging to assess. Nevertheless, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the likes of power puncher David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Hatton Homage Motivates Manchester Derby Contest
In the supporting bout, Arthur overcame Bradley Rea by divided decision in a terrific Manchester vs Manchester battle to win the European light heavyweight championship.
The arena became quiet for a deeply poignant homage prior to the bout as a bell ceremony rang in honor of local hero Ricky Hatton, who died aged 46 in last month.
It was the premier televised boxing event in the city since the passing of Hatton. As the final chime ended, the quiet became a chant of "only one Hatton".
The competition inside the ropes matched the sentiment. Arthur was knocked down by a powerful punch directly to the face in the initial stage, standing uncertainly to his position on the referee's count.
But the red-colored shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was damaged soon, significant bruising appearing under his eye as the pair landed punishing blows.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the pace seldom decreased over the full distance.
The victor edged the decision with scoring tallies of 114-114, 115-113 and wider gap, but each competitor were relentless and full of heart, exactly what their idol the legend represented.
