UK light heavyweight Joshua Buatsi made a winning yet unimpressive beginning to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a tight 10-round bout at the Manchester venue.
But the challenger landed significant shots, landing some telling individual punches in a contest where the styles never quite clicked, with excessive holding marring the action.
The scoring judges had it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my finest display but what defines champions? They win," Buatsi stated in the arena as he was greeted with disapproval from the spectators.
The Londoner returned to winning ways after his initial pro loss - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers thought Parker, thirty-one, deserved the decision but the British boxer suffered the second defeat of his 28-match career.
The Champion Earns Triumph in Slow Affair
The fighter's advancement has plateaued in the past few years through management issues and extended periods of lack of fights.
Previously predicted as the rising talent of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with both competitors willing to overcommit in a tentative early stages.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was making just his second bout at light heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for Buatsi, who worked behind a solid defense to barely take the cleaner periods.
The spectators' acceptance wore thin because of constant clinching, with the challenger falling to the floor various instances during grappling.
Parker closed well, but several stanzas were hard to judge. However, ex-champion the boxing analyst - working with the broadcast network - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor advances, but with the likes of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Hatton Memorial Motivates Local Rivalry Battle
In the co-main event, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent Manchester vs Manchester fight to capture the continental championship.
The arena grew hushed for a deeply poignant memorial ahead of the contest as a bell ceremony rang in memory of the city's native Ricky Hatton, who died at age 46 in last month.
It was the premier televised fight night in the city since Hatton's death. As the last bell ended, the stillness became a shout of "only one Ricky Hatton".
The fighting inside the ropes equaled the sentiment. Arthur was floored by a left hook directly to the jaw in the second round, standing uncertainly to his position on the count of eight.
But the red-colored shorts of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was injured soon, noticeable damage appearing under his orbital area as the fighters exchanged punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, fought fire with fire with powerful upward punches as the intensity hardly diminished over twelve rounds.
The victor barely secured the result with official scores of even score, narrow lead and wider gap, but each competitor were persistent and full of heart, everything their hero the legend represented.
