British light heavyweight Joshua Buatsi earned a win yet unimpressive start to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a competitive 10-round fight at the boxing arena.
But Parker landed significant shots, delivering effective individual punches in a fight where the techniques didn't complement each other, with frequent clinching marring the contest.
Two judges had it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my finest showing but what defines champions? They win," the victor commented in the ringside as he was faced boos from the spectators.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts believed the challenger, thirty-one, should have been awarded the victory but the Derbyshire fighter suffered the second loss of his 28-bout career.
Buatsi Claims Triumph in Uneventful Match
Buatsi's progress has plateaued in recent times through management issues and considerable time of inactivity.
Once tipped as the rising talent of British boxing, he now stands trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with both competitors prepared to be aggressive in a cagey opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after damaging his hand mid-fight, was making just his second bout at light heavyweight.
By the halfway stage, the underdog had made it competitive, causing problems for the favorite, who operated with a tight guard to edge the better periods.
The spectators' acceptance diminished because of frequent holding, with Parker going down several times during grappling.
The underdog ended stronger, but multiple periods were hard to judge. Although, previous titleholder the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just one round.
The victor advances, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still appears challenging.
Hatton Tribute Fuels Local Rivalry Contest
In the co-main event, Lyndon Arthur beat Bradley Rea by divided decision in a outstanding Manchester vs Manchester contest to capture the European light heavyweight belt.
The stadium grew hushed for a deeply poignant tribute before the fight as a bell ceremony tolled in memory of the city's native Hatton, who passed away aged 46 in September.
It was the initial significant broadcast match card in the city since Hatton's death. As the final chime faded, the silence transitioned to a shout of "there's only one Hatton".
The fighting within the ring matched the emotion. The eventual winner was sent to the canvas by a significant blow squarely on the jaw in the early round, rising shakily to his position on the count of eight.
But the Manchester United-red equipment of the fighter soon came forward again. From the other corner, the opponent - in competitive blue - was damaged early, noticeable damage forming under his face as the fighters traded punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the pace seldom decreased over the full distance.
The victor narrowly won the result with scoring tallies of 114-114, narrow lead and 115-112, but the two athletes were determined and full of heart, exactly what their hero the legend represented.
