British light heavyweight Joshua Buatsi made a winning yet disappointing debut to life under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a tight ten-round fight at the boxing arena.
But Parker landed significant shots, delivering effective single shots in a contest where the approaches never quite clicked, with constant grabbing marring the action.
The officials had it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They prevail," the victor commented in the arena as he was greeted with boos from the spectators.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Various analysts believed Parker, 31, merited the decision but the British boxer endured the second setback of his 28-bout professional journey.
The Champion Earns Triumph in Dull Encounter
Buatsi's progress has slowed in recent years through contract problems and long spells of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with both competitors willing to overcommit in a tentative initial rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after breaking his hand in the contest, was participating in his second bout at light-heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for the favorite, who utilized a protective stance to barely take the cleaner stanzas.
The audience's tolerance decreased during regular grabbing, with Parker falling to the floor various instances during clinches.
Parker ended stronger, but multiple periods were hard to judge. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Memorial Fuels Manchester Derby Contest
In the supporting bout, Lyndon Arthur beat Bradley Rea by close verdict in a excellent all-Manchester contest to secure the continental belt.
The stadium grew hushed for a extremely emotional memorial prior to the bout as ten bells sounded in remembrance of the city's native Hatton, who died 46 years old in September.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the last bell faded, the quiet became a roar of "there's only one Hatton".
The fighting within the ring matched the feeling. The eventual winner was knocked down by a significant blow flush on the face in the early round, rising shakily to his footing on the referee's count.
But the traditional red shorts of Arthur soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was marked up early, noticeable damage developing under his face as the duo landed powerful shots.
Rea, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity rarely dropped over the full distance.
Arthur edged the verdict with scoring tallies of 114-114, narrow lead and wider gap, but the two athletes were persistent and brave, everything their hero Hatton represented.
