British light heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a majority decision in a tight 10-round fight at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a fight where the techniques failed to mesh, with constant grabbing marring the action.
The scoring judges judged it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my best showing but what defines champions? They win," Buatsi commented in the ring as he was greeted with negative reactions from the crowd.
The British fighter returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers thought the challenger, 31, merited the victory but the Derbyshire fighter experienced the second loss of his 28-fight professional journey.
The Champion Secures Win in Uneventful Encounter
His career development has plateaued in the past few years through contract problems and extended periods of inactivity.
Once tipped as the rising talent of UK boxing, he now is positioned trying to achieve expectations - maybe in what appears to be a final roll of the dice - under recent support.
He started carefully, with neither fighter ready to overcommit in a tentative early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was making just his second fight at light heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for Buatsi, who operated with a solid defense to narrowly win the more effective periods.
The audience's tolerance decreased amid frequent holding, with the challenger going down multiple occasions during clinches.
Parker ended stronger, but many rounds were challenging to assess. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
Buatsi moves on, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Memorial Inspires Manchester Derby Battle
In the co-main event, Arthur beat Rea by close verdict in a excellent all-Manchester fight to capture the continental belt.
The stadium grew hushed for a extremely emotional homage before the fight as 10 bells rang in honor of the city's native Hatton, who passed away aged 46 in September.
It was the first major televised match card in the urban center since Hatton's passing. As the final chime faded, the quiet gave way to a shout of "there's only one Hatton".
The action inside the ropes matched the sentiment. The eventual winner was knocked down by a left hook flush on the chin in the second round, rising shakily to his feet on the count of eight.
But the Manchester United-red shorts of Arthur soon came forward again. From the other corner, the opponent - in opposing blue - was injured soon, noticeable damage forming under his eye as the fighters exchanged hard punches.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over the full distance.
Arthur narrowly won the decision with judges scorecards of draw, narrow lead and wider gap, but both fighters were persistent and courageous, all the qualities their hero the legend embodied.
