UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive debut to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a split decision in a tight 10-round contest at the Manchester venue.
But the challenger found success, delivering effective clean blows in a fight where the approaches didn't complement each other, with frequent clinching spoiling the fight.
The scoring judges judged it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They win," Buatsi said in the ring as he was met by negative reactions from the crowd.
The British fighter bounced back after his initial pro loss - an thrilling contest with his previous opponent in February - to earn a twentieth professional win.
Several boxing experts felt Parker, 31, merited the win but the British boxer suffered the second loss of his 28-fight career.
Buatsi Secures Triumph in Uneventful Affair
Buatsi's progress has slowed in recent years through promotional disputes and considerable time of lack of fights.
Earlier forecast as the future of UK boxing, he now finds himself trying to achieve expectations - perhaps in what seems like a last chance - under new promotional backing.
He began cautiously, with both competitors willing to take risks in a calculated initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after damaging his hand in the contest, was making just his second appearance at the higher weight class.
By the halfway stage, Parker had kept it close, making life awkward for the favorite, who utilized a protective stance to edge the better stanzas.
The crowd's patience wore thin amid constant clinching, with the challenger going down various instances during clinches.
Parker finished the stronger, but many rounds were challenging to assess. Although, former world champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just one round.
Buatsi advances, but with the presence of power puncher David Benavidez, and Russian star Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Homage Motivates All-Manchester Battle
In the supporting bout, Arthur overcame Rea by split-decision in a outstanding local rivalry fight to win the continental championship.
The arena fell silent for a deeply poignant homage before the fight as ten bells tolled in honor of local hero Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the last bell faded, the quiet gave way to a chant of "there's just one Ricky Hatton".
The fighting within the ring matched the sentiment. The eventual winner was sent to the canvas by a left hook flush on the chin in the initial stage, standing uncertainly to his position on the eight count.
But the traditional red equipment of the fighter soon came forward again. Across the ring, the opponent - in competitive blue - was damaged quickly, noticeable damage appearing under his orbital area as the fighters landed powerful shots.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over the full distance.
The victor edged the decision with scoring tallies of draw, close margin and 115-112, but both fighters were relentless and full of heart, exactly what their hero Hatton represented.
