UK light-heavyweight Joshua Buatsi secured a victory yet disappointing debut to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a majority decision in a close ten-round fight at the Manchester venue.
But the challenger landed significant shots, landing some telling individual punches in a match where the approaches never quite clicked, with constant grabbing marring the fight.
The scoring judges scored it 96-94 in favor to Buatsi, with the third 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 disapproval from the audience.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in February - to secure a 20th career victory.
Various analysts believed Parker, 31, merited the victory but the British boxer endured the second defeat of his 28-fight professional journey.
The Champion Earns Win in Uneventful Match
His career development has slowed in recent times through management issues and extended periods of lack of fights.
Once tipped as the rising talent of British boxing, he now is positioned trying to achieve expectations - maybe in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with neither fighter ready to overcommit in a calculated early stages.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was participating in his second bout at light heavyweight.
By the halfway stage, the underdog had made it competitive, making life awkward for the favorite, who operated with a protective stance to barely take the better stanzas.
The audience's tolerance diminished because of regular grabbing, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker finished the stronger, but many rounds were hard to judge. Nevertheless, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, scoring for Buatsi just one round.
Buatsi advances, but with the presence of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Memorial Inspires Local Rivalry Battle
In the secondary fight, Arthur beat Bradley Rea by close verdict in a excellent Manchester vs Manchester battle to win the European light heavyweight championship.
The venue grew hushed for a profoundly moving memorial ahead of the contest as a bell ceremony rang in honor of the city's native Hatton, who died aged 46 in recently.
It was the premier televised match card in the city since the passing of Hatton. As the final chime ended, the silence became a shout of "only one Ricky Hatton".
The action during the bout equaled the emotion. Arthur was knocked down by a left hook flush on the face in the early round, rising shakily to his position on the referee's count.
But the Manchester United-red attire of Arthur soon came forward again. On the opposite side, Rea - in opposing blue - was damaged early, significant bruising appearing under his orbital area as the pair exchanged hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the pace hardly diminished over 12 rounds.
Arthur narrowly won the verdict with official scores of even score, 115-113 and 115-112, but the two athletes were determined and full of heart, all the qualities their idol Hatton stood for.
