British light-heavyweight Joshua Buatsi earned a win yet disappointing beginning to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a split decision in a close ten-round fight at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful single shots in a match where the techniques never quite clicked, with constant grabbing disrupting the fight.
The officials judged it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my best performance but how do champions respond? They prevail," Buatsi said in the ring as he was greeted with disapproval from the spectators.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to earn a 20th pro success.
Various analysts believed Parker, thirty-one, deserved the decision but the contender suffered the second setback of his 28-bout career.
The Champion Claims Triumph in Slow Encounter
His career development has plateaued in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the future of the British fight scene, he now is positioned trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under recent support.
He started carefully, with both competitors willing to overcommit in a cagey early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after damaging his hand in the contest, was participating in his second bout at light heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who utilized a solid defense to narrowly win the more effective stanzas.
The audience's tolerance diminished during regular grabbing, with the challenger going down multiple occasions during clinches.
The underdog finished the stronger, but many rounds were difficult to score. Nevertheless, former world champion Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
The victor moves on, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Hatton Tribute Inspires All-Manchester Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by split-decision in a outstanding all-Manchester battle to capture the European light-heavyweight title.
The arena grew hushed for a deeply poignant tribute before the fight as ten bells rang in honor of local hero Ricky Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the concluding sound disappeared, the quiet became a chant of "only one Hatton".
The competition during the bout reflected the sentiment. Arthur was floored by a left hook flush on the chin in the second round, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red shorts of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was injured quickly, heavy swelling forming under his orbital area as the fighters exchanged punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over twelve rounds.
Arthur barely secured the result with official scores of even score, close margin and clearer margin, but both fighters were persistent and brave, exactly what their idol Hatton represented.
