UK light-heavyweight Joshua Buatsi earned a win yet underwhelming start to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a split decision in a competitive 10-round fight at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful single shots in a contest where the approaches failed to mesh, with frequent clinching spoiling the contest.
The scoring judges had it 96-94 to the winner, with the final judge a even score.
"It wasn't my greatest display but what do champions do? They succeed," Buatsi stated in the arena as he was met by negative reactions from the spectators.
The Londoner got back to winning after his first professional defeat - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Some ringside observers thought Parker, 31, merited the victory but the British boxer endured the second defeat of his 28-match career.
The Champion Earns Victory in Slow Affair
His career development has slowed in recent years through contract problems and extended periods of inactivity.
Previously predicted as the next star of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter prepared to overcommit in a cagey early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after suffering a hand injury in the contest, was competing in only his second appearance at light heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who operated with a tight guard to barely take the more effective rounds.
The audience's tolerance wore thin amid frequent holding, with the challenger slipping to the canvas various instances during clinches.
Parker finished the stronger, but many rounds were hard to judge. However, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Homage Fuels Manchester Derby Battle
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a excellent all-Manchester contest to capture the European light-heavyweight championship.
The arena fell silent for a extremely emotional tribute ahead of the contest as 10 bells sounded in honor of local hero Ricky Hatton, who died at age 46 in September.
It was the first major televised boxing event in the urban center since the passing of Hatton. 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 knocked down by a powerful punch directly to the jaw in the early round, rising shakily to his footing on the count of eight.
But the red-colored attire of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was marked up soon, noticeable damage appearing under his face as the fighters traded hard punches.
Rea, 27, landed power punches but the champion, thirty-four, fought fire with fire with powerful upward punches as the action rarely dropped over twelve rounds.
The victor narrowly won the decision with scoring tallies of draw, 115-113 and 115-112, but the two athletes were persistent and full of heart, all the qualities their idol Hatton represented.
