UK light heavyweight Joshua Buatsi earned a win yet unimpressive start to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a tight 10-round contest at the Manchester venue.
But Parker found success, delivering effective clean blows in a contest where the styles never quite clicked, with frequent clinching disrupting the action.
Two judges had it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my best display but what defines champions? They win," Buatsi said in the ring as he was faced boos from the audience.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in last winter - to claim a twentieth professional win.
Several boxing experts thought the challenger, 31, deserved the victory but the contender experienced the second defeat of his 28-fight career.
The Champion Secures Victory in Slow Match
His career development has slowed in recent years through management issues and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now is positioned trying to achieve expectations - maybe in what seems like a last chance - under new promotional backing.
He began cautiously, with both competitors ready to take risks in a tentative early stages.
The challenger, whose sole prior loss came at super middleweight against his former opponent in two years ago after damaging his hand mid-fight, was making just his second fight at light heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for the favorite, who operated with a tight guard to edge the better stanzas.
The audience's tolerance diminished amid frequent holding, with Parker going down multiple occasions during close exchanges.
The underdog finished the stronger, but several stanzas were difficult to score. Although, previous titleholder Carl Frampton - working with Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor moves on, but with the likes of heavy hitter Benavidez, and Russian champion Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Memorial Motivates All-Manchester Fight
In the supporting bout, Arthur beat Rea by divided decision in a excellent Manchester vs Manchester fight to win the continental belt.
The venue became quiet for a deeply poignant homage prior to the bout as a bell ceremony tolled in honor of local hero Ricky Hatton, who died aged 46 in recently.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the concluding sound ended, the quiet became a roar of "there's only one Hatton".
The competition during the bout equaled the feeling. The eventual winner was floored by a left hook squarely on the chin in the second round, getting up unsteadily to his footing on the eight count.
But the traditional red attire of Arthur soon advanced again. Across the ring, the opponent - in rival City blue - was injured soon, significant bruising forming under his eye as the duo exchanged punishing blows.
The challenger, 27, delivered effective strikes but Arthur, 34, matched the intensity with stinging uppercuts as the action seldom decreased over 12 rounds.
The victor narrowly won the result with official scores of even score, narrow lead and wider gap, but both fighters were determined and brave, everything their hero the legend stood for.
