UK light-heavyweight Joshua Buatsi earned a win yet underwhelming beginning to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a split decision in a close 10-round contest at the Co-op Live Arena.
But the challenger found success, delivering effective individual punches in a contest where the styles never quite clicked, with excessive holding disrupting the contest.
The scoring judges judged it 96-94 to the winner, with the third a 95-95.
"It wasn't my best showing but what do champions do? They prevail," Buatsi commented in the ringside as he was faced negative reactions from the crowd.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to earn a 20th career victory.
Some ringside observers believed Parker, 31, should have been awarded the decision but the Derbyshire fighter endured the second setback of his 28-fight professional journey.
The Champion Claims Win in Dull Affair
Buatsi's progress has stalled in recent years through contract problems and long spells of inactivity.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter ready to take risks in a cagey early stages.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second bout at light heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for Buatsi, who worked behind a protective stance to narrowly win the better stanzas.
The crowd's patience diminished because of frequent holding, with the challenger going down several times during clinches.
The underdog finished the stronger, but multiple periods were challenging to assess. Nevertheless, former world champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor advances, but with the caliber of power puncher David Benavidez, and Russian star Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Hatton Memorial Inspires Manchester Derby Battle
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific local rivalry battle to win the continental championship.
The arena fell silent for a profoundly moving memorial ahead of the contest as ten bells sounded in memory of local hero Ricky Hatton, who died aged 46 in September.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the last bell ended, the stillness became a chant of "only one Hatton".
The competition during the bout matched the emotion. Arthur was knocked down by a left hook directly to the chin in the early round, getting up unsteadily to his position on the count of eight.
But the red-colored equipment of Arthur soon advanced again. Across the ring, the opponent - in opposing blue - was injured soon, noticeable damage developing under his orbital area as the fighters exchanged powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, fought fire with fire with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur narrowly won the verdict with judges scorecards of even score, close margin and clearer margin, but both fighters were determined and brave, all the qualities their idol Hatton stood for.
