UK light-heavyweight Joshua Buatsi secured a victory yet underwhelming debut to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a tight ten-round fight at the Manchester venue.
But Parker found success, delivering effective individual punches in a match where the approaches never quite clicked, with constant grabbing marring the contest.
The scoring judges scored it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my greatest performance but how do champions respond? They prevail," Buatsi stated in the arena as he was greeted with boos from the audience.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in last winter - to secure a 20th career victory.
Various analysts believed the challenger, thirty-one, deserved the decision but the Derbyshire fighter experienced the second defeat of his 28-fight career.
Buatsi Secures Triumph in Uneventful Encounter
Buatsi's progress has slowed in the past few years through contract problems and extended periods of lack of fights.
Earlier forecast as the future of the British fight scene, he now finds himself trying to realize his potential - maybe in what seems like a last chance - under fresh management.
He opened tentatively, with neither fighter willing to overcommit in a cagey early stages.
The challenger, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second bout at the higher weight class.
By the fight's midpoint, the underdog had kept it close, making life awkward for Buatsi, who operated with a solid defense to edge the cleaner stanzas.
The spectators' acceptance diminished during regular grabbing, with the challenger slipping to the canvas various instances during grappling.
Parker finished the stronger, but multiple periods were challenging to assess. Although, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Homage Fuels All-Manchester Battle
In the supporting bout, Lyndon Arthur overcame Rea by divided decision in a excellent Manchester vs Manchester fight to win the continental championship.
The arena grew hushed for a deeply poignant tribute prior to the bout as 10 bells sounded in honor of the city's native Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the final chime faded, the silence transitioned to a shout of "only one Ricky Hatton".
The fighting inside the ropes equaled the sentiment. The eventual winner was knocked down by a left hook directly to the chin in the initial stage, standing uncertainly to his footing on the eight count.
But the red-colored shorts of Arthur soon came forward again. On the opposite side, Rea - in opposing blue - was damaged quickly, noticeable damage appearing under his eye as the fighters traded powerful shots.
The challenger, 27, landed power punches but Arthur, thirty-four, matched the intensity with powerful upward punches as the action seldom decreased over 12 rounds.
The victor narrowly won the decision with scoring tallies of 114-114, 115-113 and wider gap, but each competitor were persistent and brave, all the qualities their idol Hatton stood for.
