UK light heavyweight Joshua Buatsi earned a win yet disappointing beginning to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a close 10-round bout at the Co-op Live Arena.
But the challenger had his moments, landing some telling individual punches in a match where the techniques failed to mesh, with frequent clinching disrupting the action.
Two judges had it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest display but how do champions respond? They win," the victor said in the ringside as he was faced disapproval from the crowd.
The British fighter bounced back after his initial pro loss - an memorable fight with his previous opponent in February - to claim a 20th pro success.
Some ringside observers felt the challenger, 31, should have been awarded the victory but the British boxer experienced the second setback of his 28-bout professional journey.
The Champion Secures Triumph in Dull Encounter
His career development has stalled in recent years through management issues and considerable time of inactivity.
Once tipped as the rising talent of British boxing, he now stands trying to realize his potential - possibly in what appears to be a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter ready to overcommit in a cagey opening few rounds.
Parker, 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 light heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to narrowly win the better periods.
The audience's tolerance diminished amid constant clinching, with Parker falling to the floor multiple occasions during close exchanges.
Parker ended stronger, but many rounds were hard to judge. However, previous titleholder the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Memorial Fuels Local Rivalry Fight
In the co-main event, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent local rivalry contest to capture the European light heavyweight championship.
The venue fell silent for a deeply poignant homage ahead of the contest as 10 bells sounded in honor of Manchester's own Hatton, who died aged 46 in September.
It was the premier televised fight night in the urban center since Hatton's passing. As the concluding sound ended, the stillness transitioned to a roar of "only one Hatton".
The competition within the ring equaled the emotion. Arthur was floored by a powerful punch directly to the face in the initial stage, 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 opposing blue - was marked up quickly, noticeable damage forming under his face as the fighters exchanged punishing blows.
The challenger, 27, connected with strong blows but the champion, 34, matched the intensity with powerful upward punches as the action seldom decreased over twelve rounds.
The victor narrowly won the verdict with official scores of draw, close margin and 115-112, but both fighters were relentless and courageous, all the qualities their idol Hatton embodied.
