UK light heavyweight Joshua Buatsi earned a win yet disappointing debut to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a majority decision in a close ten-round bout at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful clean blows in a contest where the styles failed to mesh, with excessive holding disrupting the contest.
The officials judged it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my finest showing but what do champions do? They prevail," Buatsi stated in the ring as he was greeted with boos from the spectators.
The British fighter got back to winning after his maiden career setback - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Some ringside observers believed Parker, 31, merited the win but the British boxer suffered the second defeat of his 28-fight professional journey.
The Champion Secures Triumph in Uneventful Affair
His career development has stalled in the past few years through contract problems and considerable time of inactivity.
Previously predicted as the next star of British boxing, he now stands trying to achieve expectations - perhaps in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with both competitors prepared to take risks in a tentative early stages.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand in the contest, was making just his second fight at light heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who operated with a protective stance to barely take the better stanzas.
The audience's tolerance wore thin amid constant clinching, with the challenger going down several times during clinches.
Parker ended stronger, but several stanzas were hard to judge. Although, former world champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, giving Buatsi just a single round.
Buatsi advances, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Hatton Tribute Fuels Local Rivalry Contest
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a terrific local rivalry fight to capture the European light heavyweight belt.
The venue became quiet for a profoundly moving memorial prior to the bout as ten bells sounded in memory of local hero Ricky Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the last bell disappeared, the silence became a shout of "there's just one Ricky Hatton".
The competition within the ring equaled the sentiment. The eventual winner was floored by a significant blow directly to the chin in the second round, standing uncertainly to his feet on the eight count.
But the Manchester United-red attire of the fighter soon advanced again. On the opposite side, the opponent - in opposing blue - was damaged soon, heavy swelling forming under his eye as the fighters exchanged powerful shots.
The challenger, 27, connected with strong blows but Arthur, 34, responded in kind with effective uppercuts as the intensity hardly diminished over 12 rounds.
The victor edged the verdict with official scores of even score, narrow lead and clearer margin, but the two athletes were relentless and full of heart, everything their inspiration Hatton represented.
