UK light-heavyweight Joshua Buatsi earned a win yet underwhelming beginning to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a split decision in a close ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective clean blows in a match where the styles never quite clicked, with excessive holding marring the fight.
The scoring judges had it 96-94 for to Buatsi, with the third a even score.
"It wasn't my greatest showing but how do champions respond? They win," the victor commented in the ringside as he was greeted with boos from the crowd.
The British fighter returned to winning ways after his maiden career setback - an epic battle with Callum Smith in last winter - to earn a 20th pro success.
Various analysts thought the challenger, 31, deserved the decision but the contender experienced the second setback of his 28-match career.
The Champion Secures Win in Slow Match
His career development has plateaued in the past few years through management issues and extended periods of lack of fights.
Once tipped as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what appears to be a last chance - under fresh management.
He opened tentatively, with neither fighter prepared to be aggressive in a cagey initial rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was participating in his second fight at light-heavyweight.
By the halfway stage, Parker had kept it close, causing problems for the favorite, who utilized a solid defense to edge the more effective rounds.
The audience's tolerance diminished because of constant clinching, with the challenger going down several times during clinches.
Parker finished the stronger, but many rounds were difficult to score. Although, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, awarding Buatsi just a single round.
The victor moves on, but with the presence of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Tribute Inspires Local Rivalry Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a excellent Manchester vs Manchester contest to secure the European light heavyweight title.
The arena became quiet for a profoundly moving homage before the fight as ten bells tolled in honor of local hero Hatton, who passed away 46 years old in September.
It was the premier televised match card in the city since Hatton's death. As the final chime disappeared, the stillness transitioned to a chant of "there's only one Ricky Hatton".
The action inside the ropes reflected the emotion. Arthur was sent to the canvas by a significant blow flush on the jaw in the early round, standing uncertainly to his footing on the eight count.
But the red-colored equipment of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was damaged soon, noticeable damage appearing under his eye as the duo traded punishing blows.
The challenger, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with effective uppercuts as the intensity rarely dropped over 12 rounds.
The victor narrowly won the result with official scores of even score, narrow lead and wider gap, but each competitor were determined and courageous, exactly what their inspiration Hatton embodied.
