UK light heavyweight Joshua Buatsi made a winning yet unimpressive debut to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a tight 10-round contest at the Manchester venue.
But Parker landed significant shots, landing some telling clean blows in a match where the styles didn't complement each other, with constant grabbing spoiling the contest.
The officials judged it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my best performance but how do champions respond? They succeed," the victor commented in the ringside as he was met by negative reactions from the audience.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to secure a twentieth professional win.
Some ringside observers believed the challenger, thirty-one, deserved the decision but the British boxer endured the second loss of his 28-fight professional journey.
Buatsi Earns Victory in Dull Encounter
His career development has stalled 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 - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter ready to overcommit in a cagey early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second fight at the higher weight class.
By the halfway stage, Parker had kept it close, creating difficulties for Buatsi, who worked behind a protective stance to edge the cleaner periods.
The crowd's patience decreased amid frequent holding, with Parker going down various instances during close exchanges.
The underdog closed well, but multiple periods were challenging to assess. Although, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Hatton Tribute Inspires All-Manchester Fight
In the supporting bout, Arthur defeated Rea by divided decision in a outstanding all-Manchester contest to capture the continental title.
The venue grew hushed for a deeply poignant tribute before the fight as a bell ceremony tolled in remembrance of the city's native Hatton, who died aged 46 in recently.
It was the premier televised fight night in the urban center since the passing of Hatton. As the final chime disappeared, the quiet gave way to a shout of "there's only one Hatton".
The action during the bout reflected the emotion. The eventual winner was knocked down by a powerful punch flush on the chin in the early round, standing uncertainly to his feet on the eight count.
But the traditional red equipment of the fighter soon advanced again. On the opposite side, the opponent - in opposing blue - was injured quickly, significant bruising appearing under his face as the pair exchanged hard punches.
The challenger, 27, landed power punches but Arthur, 34, matched the intensity with effective uppercuts as the action seldom decreased over the full distance.
Arthur narrowly won the verdict with scoring tallies of 114-114, narrow lead and wider gap, but both fighters were persistent and courageous, all the qualities their inspiration the legend represented.
