UK light-heavyweight Joshua Buatsi earned a win yet underwhelming beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a majority decision in a close ten-round bout at the Manchester venue.
But Parker found success, delivering effective individual punches in a fight where the techniques failed to mesh, with constant grabbing marring the fight.
The scoring judges scored it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest display but what do champions do? They win," Buatsi said in the arena as he was faced boos from the crowd.
The Londoner bounced back after his initial pro loss - an memorable fight with his previous opponent in last winter - to secure a 20th career victory.
Various analysts thought Parker, 31, deserved the decision but the British boxer suffered the second setback of his 28-fight professional journey.
The Champion Claims Win in Slow Affair
His career development has stalled in recent times through promotional disputes and long spells of lack of fights.
Previously predicted as the future of UK boxing, he now stands trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under fresh management.
He began cautiously, with neither fighter ready to be aggressive in a calculated opening few rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in two years ago after damaging his hand during the bout, was participating in his second fight at light heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who operated with a protective stance to edge the cleaner periods.
The audience's tolerance wore thin during constant clinching, with Parker going down various instances during clinches.
The underdog ended stronger, but multiple periods were difficult to score. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the presence of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Hatton Memorial Fuels Manchester Derby Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding local rivalry contest to secure the continental title.
The venue grew hushed for a deeply poignant homage before the fight as 10 bells sounded in remembrance of the city's native Ricky Hatton, who succumbed at age 46 in recently.
It was the premier televised fight night in the urban center since Hatton's death. As the final chime disappeared, the quiet transitioned to a chant of "there's just one Ricky Hatton".
The fighting within the ring reflected the feeling. Arthur was knocked down by a powerful punch flush on the jaw in the early round, standing uncertainly to his footing on the eight count.
But the traditional red attire of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was damaged soon, noticeable damage forming under his orbital area as the fighters exchanged powerful shots.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with powerful upward punches as the action hardly diminished over 12 rounds.
The victor edged the verdict with scoring tallies of 114-114, 115-113 and clearer margin, but the two athletes were relentless and full of heart, exactly what their hero the legend represented.
