UK light heavyweight Joshua Buatsi secured a victory yet underwhelming start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a split decision in a tight ten-round bout at the boxing arena.
But Parker had his moments, landing some telling clean blows in a fight where the approaches failed to mesh, with excessive holding disrupting the fight.
The officials judged it 96-94 to the winner, with the third a even score.
"It wasn't my finest showing but how do champions respond? They prevail," the victor stated in the ring as he was faced boos from the audience.
The British fighter got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to secure a 20th pro success.
Some ringside observers felt Parker, 31, should have been awarded the win but the Derbyshire fighter suffered the second setback of his 28-bout career.
The Champion Claims Win in Uneventful Encounter
Buatsi's progress has stalled in the past few years through management issues and long spells of lack of fights.
Earlier forecast as the future of UK boxing, he now is positioned trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter willing to overcommit in a calculated initial rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after breaking his hand in the contest, was making just his second fight at light-heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a protective stance to narrowly win the better periods.
The spectators' acceptance wore thin during regular grabbing, with Parker falling to the floor multiple occasions during grappling.
The underdog closed well, but several stanzas were hard to judge. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, scoring for Buatsi just a single round.
Buatsi moves on, but with the caliber of hard hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Hatton Tribute Fuels Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific local rivalry contest to secure the European light heavyweight championship.
The arena became quiet for a profoundly moving tribute prior to the bout as ten bells sounded in memory of the city's native Ricky Hatton, who died at age 46 in last month.
It was the initial significant broadcast fight night in the city since Hatton's death. As the final chime faded, the silence transitioned to a shout of "there's just one Ricky Hatton".
The action during the bout reflected the emotion. The eventual winner was knocked down by a powerful punch directly to the face in the second round, rising shakily to his position on the referee's count.
But the red-colored attire of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was damaged soon, heavy swelling developing under his eye as the fighters landed powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace hardly diminished over 12 rounds.
Arthur barely secured the decision with scoring tallies of draw, close margin and 115-112, but the two athletes were persistent and brave, everything their hero the legend represented.
