British light heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a tight ten-round contest at the boxing arena.
But Parker found success, connecting with powerful clean blows in a contest where the approaches never quite clicked, with constant grabbing disrupting the action.
The officials scored it 96-94 for to Buatsi, with the third a even score.
"It wasn't my greatest display but how do champions respond? They win," Buatsi stated in the ringside as he was greeted with disapproval from the spectators.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to claim a twentieth professional win.
Some ringside observers thought the challenger, 31, should have been awarded the decision but the British boxer experienced the second defeat of his 28-match professional journey.
Buatsi Earns Victory in Dull Encounter
His career development has plateaued in the past few years through management issues and long spells of inactivity.
Once tipped as the future of British boxing, he now stands trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with both competitors ready to be aggressive in a calculated early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second fight at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, making life awkward for the favorite, who utilized a tight guard to barely take the more effective rounds.
The crowd's patience decreased amid constant clinching, with the challenger going down various instances during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. However, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Memorial Motivates Manchester Derby Contest
In the secondary fight, Lyndon Arthur defeated Bradley Rea by close verdict in a excellent local rivalry fight to secure the continental belt.
The stadium became quiet for a deeply poignant homage before the fight as 10 bells tolled in remembrance of the city's native Ricky Hatton, who died at age 46 in September.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the last bell faded, the stillness became a roar of "only one Ricky Hatton".
The fighting within the ring reflected the sentiment. The eventual winner was sent to the canvas by a powerful punch squarely on the face in the initial stage, rising shakily to his position on the referee's count.
But the red-colored equipment of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was marked up quickly, noticeable damage appearing under his orbital area as the fighters landed punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, matched the intensity with stinging uppercuts as the action hardly diminished over 12 rounds.
Arthur narrowly won the verdict with official scores of even score, narrow lead and 115-112, but both fighters were persistent and full of heart, all the qualities their inspiration Hatton represented.
