British light heavyweight Joshua Buatsi made a winning yet unimpressive debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a judges' verdict in a competitive 10-round bout at the boxing arena.
But the challenger landed significant shots, delivering effective individual punches in a match where the approaches didn't complement each other, with excessive holding spoiling the contest.
The officials had it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my finest display but what defines champions? They succeed," the victor said in the ringside as he was met by disapproval from the crowd.
The British fighter bounced back after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a twentieth professional win.
Several boxing experts felt the challenger, 31, merited the victory but the British boxer endured the second defeat of his 28-match professional journey.
The Champion Earns Victory in Slow Encounter
His career development has slowed in recent years through management issues and long spells of inactivity.
Previously predicted as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under recent support.
He began cautiously, with both competitors willing to take risks in a calculated initial rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, making life awkward for Buatsi, who operated with a tight guard to barely take the more effective periods.
The audience's tolerance wore thin amid regular grabbing, with Parker going down several times during clinches.
The underdog finished the stronger, but many rounds were challenging to assess. However, ex-champion the boxing analyst - working with the broadcast network - had it particularly wide for Parker, scoring for Buatsi just a single round.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Russian star Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Homage Motivates All-Manchester Fight
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a outstanding local rivalry battle to capture the European light-heavyweight belt.
The stadium fell silent for a profoundly moving memorial before the fight as ten bells sounded in remembrance of the city's native Ricky Hatton, who died 46 years old in recently.
It was the premier televised fight night in the city since Hatton's passing. As the last bell disappeared, the silence became a shout of "only one Hatton".
The action within the ring equaled the emotion. The eventual winner was floored by a significant blow directly to the jaw in the initial stage, getting up unsteadily to his position on the count of eight.
But the traditional red shorts of the fighter soon advanced again. Across the ring, Rea - in opposing blue - was injured early, noticeable damage appearing under his eye as the duo traded hard punches.
Rea, 27, connected with strong blows but Arthur, 34, matched the intensity with stinging uppercuts as the pace hardly diminished over 12 rounds.
Arthur edged the decision with scoring tallies of 114-114, 115-113 and 115-112, but the two athletes were relentless and full of heart, exactly what their hero the legend embodied.
