British 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 punch output likely gained him a judges' verdict in a close ten-round contest at the Manchester venue.
But the challenger found success, connecting with powerful single shots in a match where the approaches never quite clicked, with constant grabbing marring the contest.
Two judges scored it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my greatest showing but what do champions do? They succeed," the victor commented in the ring as he was greeted with disapproval from the crowd.
The British fighter bounced back after his initial pro loss - an thrilling contest with his previous opponent in February - to secure a twentieth professional win.
Some ringside observers felt Parker, 31, deserved the decision but the contender endured the second defeat of his 28-bout professional journey.
The Champion Earns Victory in Uneventful Affair
His career development has stalled in recent times through contract problems and long spells of inactivity.
Previously predicted as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under fresh management.
He started carefully, with both competitors willing to take risks in a calculated initial rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in two years ago after breaking his hand in the contest, was competing in only his second bout at light-heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who worked behind a solid defense to edge the better stanzas.
The spectators' acceptance wore thin because of regular grabbing, with the challenger falling to the floor various instances during clinches.
Parker ended stronger, but several stanzas were challenging to assess. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
The victor moves on, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Hatton Tribute Inspires All-Manchester Contest
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a terrific Manchester vs Manchester battle to capture the European light heavyweight title.
The stadium became quiet for a extremely emotional memorial ahead of the contest as a bell ceremony sounded in honor of the city's native Hatton, who succumbed 46 years old in last month.
It was the premier televised boxing event in the city since Hatton's death. As the concluding sound disappeared, the stillness became a roar of "there's only one Hatton".
The fighting during the bout matched the feeling. The eventual winner was knocked down by a significant blow squarely on the face in the initial stage, standing uncertainly to his footing on the count of eight.
But the traditional red attire of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was injured early, significant bruising forming under his eye as the duo landed punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, fought fire with fire with effective uppercuts as the action seldom decreased over the full distance.
The victor edged the decision with judges scorecards of 114-114, 115-113 and clearer margin, but each competitor were relentless and brave, exactly what their idol Hatton represented.
