British light-heavyweight Joshua Buatsi earned a win yet disappointing debut to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a split decision in a competitive 10-round bout at the Manchester venue.
But Parker found success, landing some telling individual punches in a fight where the techniques never quite clicked, with constant grabbing marring the fight.
The scoring judges judged it 96-94 to the winner, with the remaining official a even score.
"It wasn't my greatest display but how do champions respond? They succeed," the victor said in the ring as he was greeted with boos from the crowd.
The British fighter got back to winning after his first professional defeat - an memorable fight with his previous opponent in February - to claim a 20th career victory.
Various analysts felt Parker, 31, should have been awarded the win but the British boxer suffered the second setback of his 28-bout professional journey.
Buatsi Secures Triumph in Slow Encounter
His career development has slowed in the past few years through promotional disputes and considerable time of lack of fights.
Previously predicted as the next star of UK boxing, he now is positioned trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with neither fighter willing to be aggressive in a cagey early stages.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after suffering a hand injury during the bout, was making just his second appearance at light-heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for the favorite, who utilized a solid defense to narrowly win the better stanzas.
The spectators' acceptance decreased amid constant clinching, with Parker going down several times during clinches.
Parker ended stronger, but many rounds were hard to judge. However, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor moves on, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still seems difficult.
Hatton Memorial Fuels All-Manchester Fight
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a terrific Manchester vs Manchester fight to secure the European light heavyweight championship.
The arena fell silent for a extremely emotional tribute before the fight as 10 bells rang in memory of Manchester's own Ricky Hatton, who succumbed 46 years old in recently.
It was the first major televised boxing event in the location since the passing of Hatton. As the final chime ended, the stillness transitioned to a chant of "only one Ricky Hatton".
The action inside the ropes matched the feeling. The eventual winner was knocked down by a significant blow squarely on the chin in the second round, rising shakily to his footing on the eight count.
But the traditional red attire of Arthur soon came forward again. From the other corner, the opponent - in opposing blue - was damaged quickly, significant bruising developing under his face as the pair landed hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, fought fire with fire with stinging uppercuts as the pace hardly diminished over twelve rounds.
The victor narrowly won the result with official scores of even score, 115-113 and wider gap, but each competitor were relentless and brave, everything their idol Hatton embodied.
