UK light-heavyweight Joshua Buatsi earned a win yet disappointing debut to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a competitive 10-round contest at the boxing arena.
But Parker had his moments, landing some telling single shots in a fight where the styles failed to mesh, with frequent clinching marring the action.
The scoring judges scored it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my greatest showing but what do champions do? They prevail," the victor commented in the ring as he was greeted with boos from the audience.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in February - to claim a twentieth professional win.
Various analysts believed Parker, 31, deserved the decision but the British boxer suffered the second loss of his 28-match professional journey.
The Champion Earns Win in Slow Affair
The fighter's advancement has plateaued in the past few years through promotional disputes and considerable time of lack of fights.
Previously predicted as the next star of British boxing, he now stands trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under fresh management.
He began cautiously, with the two boxers prepared to overcommit in a tentative opening few rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in two years ago after suffering a hand injury during the bout, was participating in his second fight at light heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who worked behind a solid defense to narrowly win the more effective stanzas.
The spectators' acceptance wore thin because of constant clinching, with the challenger going down multiple occasions during grappling.
Parker finished the stronger, but several stanzas were challenging to assess. Nevertheless, former world champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
The victor moves on, but with the likes of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Homage Motivates Manchester Derby Battle
In the supporting bout, Arthur defeated Bradley Rea by close verdict in a outstanding local rivalry battle to secure the European light heavyweight title.
The stadium became quiet for a extremely emotional homage ahead of the contest as ten bells rang in memory of the city's native Ricky Hatton, who died 46 years old in September.
It was the premier televised boxing event in the city since Hatton's death. As the last bell ended, the stillness transitioned to a roar of "there's only one Hatton".
The competition within the ring matched the sentiment. Arthur was sent to the canvas by a powerful punch squarely on the chin in the initial stage, standing uncertainly to his footing on the referee's count.
But the Manchester United-red shorts of Arthur soon came forward again. On the opposite side, the opponent - in opposing blue - was damaged early, heavy swelling forming under his orbital area as the fighters exchanged punishing blows.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the pace rarely dropped over 12 rounds.
Arthur edged the decision with judges scorecards of draw, close margin and 115-112, but both fighters were relentless and courageous, exactly what their inspiration the legend embodied.
