UK light heavyweight Joshua Buatsi earned a win yet unimpressive beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a tight 10-round bout at the Co-op Live Arena.
But the challenger had his moments, delivering effective single shots in a fight where the styles failed to mesh, with excessive holding marring the action.
Two judges had it 96-94 for to Buatsi, with the third a even score.
"It wasn't my greatest performance but how do champions respond? They win," the victor commented in the ringside as he was greeted with disapproval from the audience.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in last winter - to earn a twentieth professional win.
Some ringside observers believed Parker, thirty-one, merited the win but the contender suffered the second defeat of his 28-fight career.
Buatsi Earns Win in Uneventful Encounter
His career development has slowed in recent times through promotional disputes and extended periods of lack of fights.
Earlier forecast as the next star of British boxing, he now is positioned trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with both competitors willing to be aggressive in a calculated initial rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after breaking his hand mid-fight, was making just his second appearance at light heavyweight.
By the middle rounds, the underdog had made it competitive, making life awkward for the favorite, who utilized a tight guard to barely take the cleaner periods.
The spectators' acceptance wore thin because of regular grabbing, with the challenger slipping to the canvas several times during clinches.
Parker closed well, but many rounds were difficult to score. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, awarding Buatsi just one round.
The victor moves on, but with the presence of hard hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Hatton Tribute Fuels All-Manchester Contest
In the secondary fight, Arthur beat Rea by divided decision in a outstanding Manchester vs Manchester battle to win the continental championship.
The stadium fell silent for a extremely emotional tribute ahead of the contest as 10 bells tolled in memory of local hero Ricky Hatton, who died aged 46 in recently.
It was the premier televised match card in the city since Hatton's death. As the final chime disappeared, the quiet transitioned to a chant of "there's just one Hatton".
The action inside the ropes matched the sentiment. Arthur was floored by a left hook flush on the jaw in the initial stage, rising shakily to his feet on the referee's count.
But the Manchester United-red attire of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was damaged quickly, noticeable damage developing under his eye as the duo landed punishing blows.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, matched the intensity with powerful upward punches as the intensity hardly diminished over twelve rounds.
Arthur edged the verdict with scoring tallies of 114-114, narrow lead and 115-112, but both fighters were persistent and courageous, all the qualities their inspiration Hatton represented.
