British light-heavyweight Joshua Buatsi made a winning yet unimpressive beginning to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a split decision in a tight 10-round fight at the boxing arena.
But Parker had his moments, delivering effective clean blows in a fight where the techniques never quite clicked, with excessive holding spoiling the fight.
Two judges judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest showing but what defines champions? They prevail," the victor said in the ringside as he was faced disapproval from the crowd.
The Londoner got back to winning after his initial pro loss - an memorable fight with his previous opponent in February - to secure a twentieth professional win.
Several boxing experts believed Parker, 31, deserved the decision but the Derbyshire fighter endured the second setback of his 28-bout professional journey.
The Champion Earns Victory in Uneventful Match
The fighter's advancement has plateaued in recent years through management issues and long spells of lack of fights.
Earlier forecast as the future of UK boxing, he now stands trying to achieve expectations - perhaps in what appears to be a last chance - under recent support.
He began cautiously, with both competitors willing to be aggressive in a calculated early stages.
Parker, whose single earlier setback came at the lower weight class against his former opponent in two years ago 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 stayed in contention, making life awkward for Buatsi, who utilized a tight guard to narrowly win the better stanzas.
The crowd's patience decreased amid regular grabbing, with the challenger falling to the floor various instances during grappling.
Parker closed well, but several stanzas were hard to judge. Although, ex-champion the boxing analyst - working with Dazn - had it particularly wide for the challenger, giving Buatsi just one round.
The victor progresses, but with the caliber of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Homage Inspires Local Rivalry Battle
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a terrific local rivalry fight to win the European light heavyweight championship.
The arena fell silent for a extremely emotional memorial prior to the bout as 10 bells rang in honor of local hero Ricky Hatton, who died 46 years old in last month.
It was the first major televised fight night in the urban center since Hatton's death. As the last bell faded, the silence gave way to a shout of "there's just one Ricky Hatton".
The fighting during the bout equaled the emotion. The eventual winner was floored by a powerful punch directly to the jaw in the second round, standing uncertainly to his footing on the count of eight.
But the Manchester United-red equipment of Arthur soon pressed ahead again. From the other corner, Rea - in competitive blue - was damaged soon, significant bruising developing under his orbital area as the pair exchanged hard punches.
Rea, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur edged the verdict with official scores of draw, close margin and 115-112, but both fighters were relentless and courageous, exactly what their inspiration the legend stood for.
