UK light-heavyweight Joshua Buatsi earned a win yet unimpressive debut to his career under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a close 10-round contest at the Co-op Live Arena.
But the challenger found success, connecting with powerful clean blows in a fight where the techniques never quite clicked, with excessive holding marring the fight.
The scoring judges judged it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my greatest display but what do champions do? They win," Buatsi commented in the ring as he was faced disapproval from the spectators.
The Londoner bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a 20th pro success.
Some ringside observers felt Parker, 31, should have been awarded the win but the British boxer experienced the second defeat of his 28-match professional journey.
The Champion Earns Victory in Dull Match
His career development has slowed in recent years through promotional disputes and considerable time of lack of fights.
Once tipped as the next star of the British fight scene, he now stands trying to achieve expectations - perhaps in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with both competitors willing to overcommit in a tentative opening few rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after suffering a hand injury in the contest, was participating in his second fight at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for the favorite, who utilized a protective stance to narrowly win the better periods.
The audience's tolerance wore thin amid frequent holding, with the challenger going down multiple occasions during grappling.
The underdog finished the stronger, but many rounds were hard to judge. Nevertheless, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for the challenger, scoring for Buatsi just one round.
Buatsi progresses, but with the presence of power puncher David Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Hatton Homage Fuels Manchester Derby Battle
In the secondary fight, Lyndon Arthur beat Bradley Rea by split-decision in a outstanding all-Manchester battle to capture the continental championship.
The stadium fell silent for a extremely emotional memorial prior to the bout as a bell ceremony tolled in memory of the city's native Ricky Hatton, who died aged 46 in recently.
It was the initial significant broadcast match card in the location since Hatton's death. As the last bell ended, the stillness transitioned to a shout of "only one Ricky Hatton".
The fighting during the bout matched the emotion. The eventual winner was knocked down by a significant blow flush on the chin in the second round, rising shakily to his position on the referee's count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was injured quickly, significant bruising developing under his eye as the duo exchanged powerful shots.
Rea, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over the full distance.
Arthur edged the decision with scoring tallies of 114-114, close margin and 115-112, but each competitor were persistent and brave, exactly what their inspiration Hatton represented.
