British light-heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a tight ten-round bout at the boxing arena.
But the challenger had his moments, landing some telling clean blows in a fight where the techniques failed to mesh, with excessive holding disrupting the contest.
Two judges had it 96-94 for to Buatsi, with the third a even score.
"It wasn't my best performance but how do champions respond? They win," the victor commented in the ringside as he was met by boos from the audience.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Various analysts felt the challenger, 31, deserved the victory but the contender experienced the second setback of his 28-match career.
The Champion Secures Victory in Dull Affair
His career development has slowed in the past few years through management issues and considerable time of lack of fights.
Once tipped as the next star of British boxing, he now stands trying to realize his potential - possibly in what feels like a last chance - under recent support.
He began cautiously, with the two boxers prepared to take risks in a tentative early stages.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after breaking his hand mid-fight, was making just his second appearance at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a tight guard to narrowly win the cleaner rounds.
The audience's tolerance diminished during regular grabbing, with the challenger falling to the floor multiple occasions during clinches.
Parker finished the stronger, but several stanzas were hard to judge. Although, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Hatton Tribute Fuels All-Manchester Contest
In the secondary fight, Arthur overcame Rea by divided decision in a terrific Manchester vs Manchester battle to capture the European light-heavyweight championship.
The venue fell silent for a profoundly moving tribute prior to the bout as a bell ceremony tolled in remembrance of the city's native Ricky Hatton, who passed away 46 years old in recently.
It was the first major televised match card in the location since the passing of Hatton. As the concluding sound disappeared, the quiet became a shout of "only one Ricky Hatton".
The action during the bout matched the emotion. The eventual winner was knocked down by a significant blow flush on the jaw in the second round, getting up unsteadily to his position on the eight count.
But the traditional red equipment of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was marked up soon, noticeable damage forming under his face as the duo exchanged hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with effective uppercuts as the intensity rarely dropped over 12 rounds.
Arthur edged the decision with scoring tallies of 114-114, close margin and 115-112, but both fighters were determined and full of heart, everything their inspiration the legend embodied.
