British light heavyweight Joshua Buatsi made a winning yet unimpressive debut to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a competitive ten-round fight at the Manchester venue.
But the challenger landed significant shots, connecting with powerful individual punches in a fight where the approaches never quite clicked, with constant grabbing marring the action.
The scoring judges judged it 96-94 to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest performance but what defines champions? They win," the victor said in the ring as he was faced disapproval from the crowd.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in February - to secure a 20th career victory.
Several boxing experts thought Parker, 31, deserved the decision but the British boxer experienced the second loss of his 28-fight career.
Buatsi Claims Triumph in Slow Encounter
His career development has stalled in the past few years through contract problems and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now stands trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with both competitors willing to take risks in a cagey initial rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was making just his second appearance at light-heavyweight.
By the halfway stage, Parker had made it competitive, making life awkward for the favorite, who utilized a protective stance to edge the cleaner periods.
The spectators' acceptance wore thin because of regular grabbing, with Parker going down multiple occasions during grappling.
Parker finished the stronger, but several stanzas were difficult to score. Although, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, giving Buatsi just one round.
The victor advances, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - based on this performance - still appears challenging.
Hatton Tribute Inspires Local Rivalry Fight
In the co-main event, Lyndon Arthur beat Bradley Rea by split-decision in a terrific local rivalry battle to capture the continental belt.
The stadium grew hushed for a extremely emotional memorial ahead of the contest as ten bells rang in remembrance of the city's native Hatton, who passed away at age 46 in September.
It was the initial significant broadcast match card in the city since Hatton's passing. As the last bell disappeared, the silence became a roar of "there's only one Ricky Hatton".
The competition within the ring equaled the sentiment. Arthur was floored by a powerful punch directly to the jaw in the initial stage, standing uncertainly to his position on the count of eight.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, the opponent - in opposing blue - was injured soon, significant bruising appearing under his eye as the fighters exchanged hard punches.
Rea, twenty-seven, landed power punches but the champion, 34, fought fire with fire with powerful upward punches as the pace rarely dropped over the full distance.
The victor barely secured the result with official scores of 114-114, close margin and 115-112, but both fighters were determined and brave, everything their inspiration the legend represented.
