UK light heavyweight Joshua Buatsi earned a win yet unimpressive beginning to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a majority decision in a competitive 10-round fight at the boxing arena.
But Parker found success, connecting with powerful single shots in a match where the techniques failed to mesh, with excessive holding disrupting the action.
Two judges scored it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest display but what defines champions? They win," the victor stated in the ringside as he was greeted with negative reactions from the spectators.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts believed Parker, thirty-one, deserved the victory but the contender suffered the second loss of his 28-bout career.
The Champion Secures Win in Uneventful Match
His career development has plateaued in the past few years through contract problems and considerable time of inactivity.
Once tipped as the rising talent of British boxing, he now stands trying to fulfil that early promise - possibly in what appears to be a final roll of the dice - under fresh management.
He started carefully, with both competitors ready to overcommit in a cagey early stages.
Parker, whose single earlier setback came at the lower weight class against John Ryder in two years ago after suffering a hand injury during the bout, was competing in only his second fight at the higher weight class.
By the halfway stage, Parker had made it competitive, causing problems for the favorite, who operated with a protective stance to narrowly win the better stanzas.
The audience's tolerance wore thin amid frequent holding, with Parker slipping to the canvas various instances during grappling.
Parker ended stronger, but many rounds were hard to judge. However, previous titleholder Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Tribute Motivates All-Manchester Contest
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding all-Manchester contest to capture the European light heavyweight belt.
The venue grew hushed for a deeply poignant tribute before the fight as ten bells rang in memory of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the first major televised fight night in the city since the passing of Hatton. As the final chime faded, the quiet gave way to a chant of "there's just one Ricky Hatton".
The fighting during the bout reflected the sentiment. The eventual winner was sent to the canvas by a significant blow directly to the chin in the initial stage, getting up unsteadily to his position on the count of eight.
But the red-colored shorts of Arthur soon came forward again. From the other corner, Rea - in competitive blue - was marked up quickly, noticeable damage forming under his eye as the pair traded punishing blows.
Rea, 27, connected with strong blows but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity rarely dropped over 12 rounds.
Arthur barely secured the result with official scores of draw, 115-113 and wider gap, but both fighters were persistent and brave, exactly what their idol Hatton represented.
