UK light heavyweight Joshua Buatsi made a winning yet underwhelming start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a close 10-round contest at the Manchester venue.
But the challenger found success, connecting with powerful individual punches in a fight where the techniques never quite clicked, with constant grabbing disrupting the fight.
The officials had it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my finest showing but what defines champions? They prevail," Buatsi stated in the ringside as he was faced boos from the audience.
The Londoner got back to winning after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a 20th career victory.
Several boxing experts believed Parker, 31, deserved the decision but the contender experienced the second defeat of his 28-bout professional journey.
The Champion Claims Victory in Uneventful Match
His career development has slowed in recent years through management issues and extended periods of inactivity.
Once tipped as the future of UK boxing, he now is positioned trying to achieve expectations - perhaps in what appears to be a last chance - under fresh management.
He started carefully, with neither fighter willing to overcommit in a tentative early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second fight at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for Buatsi, who worked behind a solid defense to narrowly win the cleaner periods.
The spectators' acceptance wore thin amid constant clinching, with Parker slipping to the canvas various instances during clinches.
Parker closed well, but many rounds were hard to judge. Although, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Tribute Motivates All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Bradley Rea by close verdict in a excellent local rivalry contest to secure the European light heavyweight belt.
The stadium fell silent for a extremely emotional homage before the fight as ten bells rang in honor of local hero Ricky Hatton, who passed away aged 46 in recently.
It was the first major televised boxing event in the city since Hatton's death. As the concluding sound ended, the silence transitioned to a chant of "there's just one Ricky Hatton".
The fighting during the bout equaled the emotion. The eventual winner was sent to the canvas by a left hook directly to the chin in the initial stage, rising shakily to his position on the eight count.
But the traditional red attire of the fighter soon advanced again. Across the ring, the opponent - in opposing blue - was injured quickly, heavy swelling forming under his face as the fighters traded powerful shots.
Rea, twenty-seven, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the action hardly diminished over twelve rounds.
The victor narrowly won the decision with scoring tallies of even score, 115-113 and 115-112, but both fighters were determined and courageous, all the qualities their hero Hatton stood for.
