British light-heavyweight Joshua Buatsi secured a victory yet disappointing start to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a tight ten-round fight at the Manchester venue.
But the challenger landed significant shots, delivering effective clean blows in a match where the styles failed to mesh, with frequent clinching disrupting the action.
Two judges had it 96-94 for to the winner, with the third a even score.
"It wasn't my finest showing but how do champions respond? They win," the victor stated in the ringside as he was greeted with disapproval from the crowd.
The British fighter returned to winning ways after his first professional defeat - an epic battle with Callum Smith in February - to earn a 20th career victory.
Several boxing experts believed the challenger, thirty-one, deserved the victory but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
Buatsi Earns Triumph in Dull Encounter
The fighter's advancement has stalled in recent years through contract problems and long spells of inactivity.
Earlier forecast as the next star of UK boxing, he now finds himself trying to realize his potential - possibly in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers ready to be aggressive in a cagey opening few rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was participating in his second bout at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for the favorite, who operated with a tight guard to edge the more effective stanzas.
The audience's tolerance diminished during constant clinching, with the challenger going down various instances during grappling.
Parker finished the stronger, but many rounds were challenging to assess. However, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just a single round.
The victor advances, but with the caliber of power puncher Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Homage Motivates Manchester Derby Battle
In the secondary fight, Arthur overcame Rea by divided decision in a outstanding all-Manchester battle to secure the European light heavyweight belt.
The arena grew hushed for a deeply poignant memorial before the fight as ten bells tolled in honor of the city's native Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised fight night in the city since Hatton's death. As the concluding sound disappeared, the silence became a shout of "there's only one Hatton".
The action inside the ropes matched the sentiment. Arthur was sent to the canvas by a left hook flush on the chin in the early round, standing uncertainly to his feet on the referee's count.
But the traditional red attire of the fighter soon advanced again. Across the ring, Rea - in rival City blue - was injured soon, noticeable damage appearing under his eye as the pair traded punishing blows.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, matched the intensity with effective uppercuts as the intensity rarely dropped over the full distance.
The victor edged the verdict with scoring tallies of even score, narrow lead and wider gap, but each competitor were persistent and courageous, all the qualities their hero Hatton represented.
