British light heavyweight Joshua Buatsi secured a victory yet underwhelming debut to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a competitive 10-round contest at the Manchester venue.
But Parker landed significant shots, connecting with powerful clean blows in a fight where the approaches didn't complement each other, with constant grabbing marring the contest.
The scoring judges had it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest showing but what defines champions? They prevail," the victor said in the ringside as he was faced negative reactions from the spectators.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a twentieth professional win.
Several boxing experts felt Parker, 31, should have been awarded the win but the contender endured the second defeat of his 28-match career.
The Champion Claims Victory in Slow Encounter
Buatsi's progress has stalled in recent years through contract problems and considerable time of inactivity.
Once tipped as the future of UK boxing, he now is positioned trying to realize his potential - perhaps in what feels like a last chance - under recent support.
He started carefully, with both competitors prepared to be aggressive in a cagey initial rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second fight at light-heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for Buatsi, who worked behind a tight guard to edge the cleaner rounds.
The crowd's patience wore thin amid frequent holding, with the challenger falling to the floor various instances during close exchanges.
The underdog ended stronger, but several stanzas were challenging to assess. Although, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for the challenger, giving Buatsi just one round.
The victor moves on, but with the likes of power puncher Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - based on this performance - still appears challenging.
Hatton Homage Motivates All-Manchester Contest
In the secondary fight, Arthur beat Bradley Rea by divided decision in a excellent all-Manchester fight to secure the European light heavyweight belt.
The arena fell silent for a profoundly moving homage prior to the bout as a bell ceremony sounded in remembrance of local hero Ricky Hatton, who died 46 years old in recently.
It was the premier televised fight night in the city since Hatton's passing. As the final chime disappeared, the stillness transitioned to a chant of "only one Hatton".
The fighting inside the ropes matched the emotion. Arthur was floored by a left hook squarely on the chin in the early round, getting up unsteadily to his feet on the eight count.
But the red-colored equipment of the fighter soon came forward again. From the other corner, the opponent - in rival City blue - was injured soon, significant bruising forming under his orbital area as the pair exchanged punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with stinging uppercuts as the intensity seldom decreased over the full distance.
Arthur narrowly won the decision with scoring tallies of draw, close margin and clearer margin, but both fighters were determined and courageous, everything their inspiration the legend represented.
