British light heavyweight Joshua Buatsi secured a victory yet disappointing beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a tight ten-round contest at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling individual punches in a fight where the styles failed to mesh, with constant grabbing disrupting the contest.
The officials scored it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my finest showing but what do champions do? They win," the victor said in the arena as he was greeted with boos from the spectators.
The Londoner got back to winning after his maiden career setback - an memorable fight with his previous opponent in last winter - to earn a 20th career victory.
Several boxing experts thought Parker, thirty-one, deserved the victory but the British boxer experienced the second loss of his 28-match career.
The Champion Earns Triumph in Uneventful Match
His career development has plateaued in recent years through contract problems and extended periods of inactivity.
Previously predicted as the future of British boxing, he now stands trying to achieve expectations - maybe in what seems like a ultimate opportunity - under fresh management.
He opened tentatively, with both competitors willing to take risks in a cagey opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was competing in only his second appearance at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a protective stance to narrowly win the cleaner stanzas.
The crowd's patience diminished during regular grabbing, with Parker falling to the floor several times during grappling.
Parker closed well, but many rounds were hard to judge. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Homage Inspires Manchester Derby Fight
In the supporting bout, Lyndon Arthur overcame Rea by close verdict in a outstanding all-Manchester contest to secure the European light heavyweight championship.
The venue fell silent for a extremely emotional tribute ahead of the contest as a bell ceremony tolled in memory of the city's native Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the concluding sound disappeared, the stillness transitioned to a roar of "there's just one Hatton".
The fighting during the bout equaled the sentiment. The eventual winner was floored by a left hook squarely on the face in the initial stage, getting up unsteadily to his position on the referee's count.
But the red-colored shorts of the fighter soon advanced again. From the other corner, the opponent - in opposing blue - was injured quickly, noticeable damage developing under his eye as the pair traded powerful shots.
Rea, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the action seldom decreased over twelve rounds.
Arthur edged the result with official scores of 114-114, 115-113 and 115-112, but both fighters were persistent and brave, everything their hero the legend stood for.
