British light-heavyweight Joshua Buatsi made a winning yet disappointing start to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a close ten-round contest at the Co-op Live Arena.
But the challenger found success, delivering effective individual punches in a fight where the techniques failed to mesh, with excessive holding spoiling the action.
The officials judged it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest performance but how do champions respond? They win," Buatsi said in the ringside as he was greeted with negative reactions from the spectators.
The British fighter got back to winning after his first professional defeat - an memorable fight with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers thought Parker, thirty-one, merited the victory but the Derbyshire fighter endured the second defeat of his 28-fight professional journey.
The Champion Claims Triumph in Dull Encounter
His career development has stalled in recent times through contract problems and long spells of inactivity.
Previously predicted as the next star of British boxing, he now finds himself trying to realize his potential - maybe in what feels like a last chance - under new promotional backing.
He started carefully, with the two boxers prepared to be aggressive in a tentative early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was making just his second bout at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, making life awkward for Buatsi, who utilized a protective stance to edge the more effective periods.
The spectators' acceptance decreased amid regular grabbing, with Parker falling to the floor several times during close exchanges.
The underdog finished the stronger, but many rounds were hard to judge. Although, ex-champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi moves on, but with the likes of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Homage Inspires All-Manchester Fight
In the secondary fight, Arthur overcame Rea by divided decision in a terrific all-Manchester fight to capture the European light-heavyweight belt.
The arena became quiet for a profoundly moving homage prior to the bout as a bell ceremony rang in honor of the city's native Hatton, who died 46 years old in September.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the final chime faded, the silence gave way to a shout of "only one Hatton".
The competition during the bout matched the feeling. Arthur was sent to the canvas by a significant blow flush on the face in the early round, getting up unsteadily to his position on the count of eight.
But the Manchester United-red shorts of Arthur soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was damaged early, significant bruising forming under his face as the duo exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with effective uppercuts as the action rarely dropped over the full distance.
The victor edged the verdict with judges scorecards of 114-114, narrow lead and clearer margin, but each competitor were determined and full of heart, all the qualities their idol Hatton stood for.
