UK light heavyweight Joshua Buatsi secured a victory yet underwhelming debut to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a competitive ten-round bout at the Co-op Live Arena.
But the challenger had his moments, landing some telling individual punches in a contest where the styles failed to mesh, with frequent clinching spoiling the fight.
Two judges had it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my best showing but what defines champions? They prevail," the victor said in the ring as he was greeted with boos from the audience.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in last winter - to earn a 20th career victory.
Various analysts thought Parker, 31, should have been awarded the win but the British boxer experienced the second setback of his 28-match career.
The Champion Secures Victory in Uneventful Encounter
The fighter's advancement has plateaued in the past few years through contract problems and considerable time of lack of fights.
Once tipped as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - maybe in what seems like a last chance - under new promotional backing.
He opened tentatively, with neither fighter willing to be aggressive in a tentative early stages.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second fight at light heavyweight.
By the halfway stage, Parker had stayed in contention, causing problems for Buatsi, who utilized a tight guard to narrowly win the more effective periods.
The audience's tolerance decreased amid regular grabbing, with Parker falling to the floor multiple occasions during clinches.
The underdog ended stronger, but several stanzas were challenging to assess. However, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
Buatsi moves on, but with the presence of power puncher David Benavidez, and Russian champion Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Tribute Motivates Manchester Derby Battle
In the secondary fight, Lyndon Arthur overcame Bradley Rea by close verdict in a outstanding local rivalry battle to win the European light heavyweight belt.
The stadium became quiet for a profoundly moving homage ahead of the contest as a bell ceremony rang in remembrance of Manchester's own Ricky Hatton, who died aged 46 in September.
It was the premier televised boxing event in the location since Hatton's death. As the last bell faded, the silence transitioned to a roar of "there's just one Ricky Hatton".
The fighting within the ring matched the sentiment. The eventual winner was floored by a powerful punch squarely on the jaw in the initial stage, standing uncertainly to his position on the eight count.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was injured soon, significant bruising forming under his eye as the duo landed hard punches.
The challenger, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with powerful upward punches as the intensity rarely dropped over the full distance.
Arthur narrowly won the result with judges scorecards of 114-114, close margin and wider gap, but the two athletes were persistent and full of heart, all the qualities their idol Hatton embodied.
