British light heavyweight Joshua Buatsi secured a victory yet underwhelming debut to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a tight 10-round fight at the Manchester venue.
But the challenger landed significant shots, delivering effective single shots in a match where the techniques didn't complement each other, with excessive holding disrupting the contest.
Two judges scored it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my best performance but what do champions do? They win," the victor commented in the ringside as he was greeted with negative reactions from the spectators.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts believed Parker, 31, deserved the win but the Derbyshire fighter suffered the second defeat of his 28-bout career.
Buatsi Earns Victory in Dull Encounter
His career development has slowed in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the future of the British fight scene, he now finds himself trying to realize his potential - possibly in what feels like a last chance - under fresh management.
He began cautiously, with neither fighter ready to take risks in a tentative opening few rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was participating in his second bout at light-heavyweight.
By the middle rounds, the underdog had made it competitive, causing problems for the favorite, who operated with a tight guard to narrowly win the cleaner periods.
The crowd's patience decreased during constant clinching, with the challenger falling to the floor several times during close exchanges.
The underdog ended stronger, but many rounds were challenging to assess. Although, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
The victor moves on, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Hatton Homage Inspires Manchester Derby Contest
In the supporting bout, Lyndon Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester contest to secure the European light heavyweight belt.
The venue grew hushed for a extremely emotional memorial before the fight as ten bells rang in memory of the city's native Ricky Hatton, who died aged 46 in last month.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the final chime ended, the stillness gave way to a roar of "only one Ricky Hatton".
The competition inside the ropes matched the emotion. The eventual winner was sent to the canvas by a powerful punch squarely on the chin in the second round, standing uncertainly to his feet on the referee's count.
But the Manchester United-red equipment of Arthur soon advanced again. Across the ring, Rea - in opposing blue - was injured early, significant bruising developing under his eye as the pair traded punishing blows.
The challenger, 27, delivered effective strikes but Arthur, 34, responded in kind with powerful upward punches as the intensity rarely dropped over twelve rounds.
Arthur edged the verdict with scoring tallies of draw, 115-113 and 115-112, but the two athletes were persistent and full of heart, exactly what their idol Hatton embodied.
