British light-heavyweight Joshua Buatsi made a winning yet unimpressive start to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a competitive ten-round contest at the Manchester venue.
But Parker landed significant shots, delivering effective individual punches in a match where the approaches never quite clicked, with frequent clinching disrupting the fight.
The scoring judges judged it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest display but how do champions respond? They win," Buatsi stated in the arena as he was met by negative reactions from the crowd.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts thought Parker, 31, should have been awarded the win but the British boxer experienced the second defeat of his 28-match career.
The Champion Earns Victory in Slow Match
Buatsi's progress has stalled in the past few years through management issues and long spells of inactivity.
Previously predicted as the rising talent of UK boxing, he now stands trying to realize his potential - possibly in what seems like a last chance - under fresh management.
He started carefully, with neither fighter willing to overcommit in a calculated early stages.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand mid-fight, was participating in his second bout at light heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for Buatsi, who utilized a protective stance to edge the cleaner stanzas.
The crowd's patience diminished amid frequent holding, with the challenger slipping to the canvas multiple occasions during clinches.
Parker closed well, but many rounds were challenging to assess. However, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the likes of power puncher Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Hatton Memorial Fuels Manchester Derby Contest
In the co-main event, Arthur defeated Bradley Rea by close verdict in a terrific Manchester vs Manchester battle to capture the European light heavyweight championship.
The stadium fell silent for a extremely emotional memorial before the fight as ten bells sounded in honor of Manchester's own Hatton, who died at age 46 in recently.
It was the first major televised fight night in the city since Hatton's death. As the last bell ended, the quiet became a roar of "there's only one Ricky Hatton".
The competition during the bout reflected the sentiment. Arthur was sent to the canvas by a left hook squarely on the jaw in the initial stage, standing uncertainly to his footing on the referee's count.
But the red-colored attire of the fighter soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was damaged quickly, noticeable damage appearing under his eye as the fighters landed powerful shots.
Rea, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with effective uppercuts as the pace rarely dropped over the full distance.
Arthur narrowly won the decision with scoring tallies of even score, close margin and wider gap, but the two athletes were relentless and full of heart, exactly what their inspiration the legend stood for.
