UK light-heavyweight Joshua Buatsi made a winning yet disappointing debut to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a majority decision in a close 10-round contest at the Co-op Live Arena.
But Parker found success, connecting with powerful single shots in a contest where the styles never quite clicked, with frequent clinching disrupting the fight.
The officials judged it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my greatest performance but how do champions respond? They prevail," the victor commented in the arena as he was faced negative reactions from the audience.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in last winter - to claim a twentieth professional win.
Various analysts believed Parker, thirty-one, should have been awarded the decision but the British boxer experienced the second loss of his 28-bout professional journey.
The Champion Earns Win in Slow Encounter
The fighter's advancement has stalled in recent years through management issues and long spells of lack of fights.
Once tipped as the rising talent of the British fight scene, he now is positioned trying to realize his potential - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers prepared to overcommit in a cagey opening few rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after damaging his hand in the contest, was participating in his second fight at light-heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for the favorite, who utilized a protective stance to barely take the better periods.
The audience's tolerance wore thin because of constant clinching, with the challenger falling to the floor multiple occasions during grappling.
Parker closed well, but multiple periods were challenging to assess. Although, former world champion the boxing analyst - working with Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the likes of power puncher David Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Hatton Memorial Inspires Local Rivalry Battle
In the supporting bout, Lyndon Arthur beat Bradley Rea by close verdict in a excellent Manchester vs Manchester fight to secure the European light-heavyweight title.
The venue grew hushed for a deeply poignant homage prior to the bout as 10 bells rang in remembrance of the city's native Hatton, who succumbed at age 46 in recently.
It was the first major televised match card in the location since the passing of Hatton. As the concluding sound ended, the silence became a chant of "only one Hatton".
The action during the bout matched the sentiment. Arthur was knocked down by a powerful punch directly to the face in the second round, rising shakily to his position on the count of eight.
But the traditional red attire of the fighter soon advanced again. On the opposite side, Rea - in competitive blue - was injured early, heavy swelling forming under his face as the duo exchanged powerful shots.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over 12 rounds.
The victor narrowly won the decision with judges scorecards of even score, close margin and wider gap, but each competitor were persistent and full of heart, everything their hero the legend represented.
