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 work rate seemingly secured him a judges' verdict in a competitive 10-round bout at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful individual punches in a contest where the techniques failed to mesh, with constant grabbing spoiling the action.
The officials judged it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my greatest showing but what defines champions? They win," the victor commented in the ringside as he was greeted with negative reactions from the spectators.
The British fighter got back to winning after his first professional defeat - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Some ringside observers believed Parker, thirty-one, should have been awarded the win but the Derbyshire fighter suffered the second setback of his 28-fight professional journey.
The Champion Claims Triumph in Dull Encounter
The fighter's advancement has plateaued in recent times through promotional disputes and extended periods of lack of fights.
Once tipped as the rising talent of UK boxing, he now finds himself trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with both competitors ready to take risks in a cagey early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second appearance at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, causing problems for Buatsi, who utilized a solid defense to barely take the better stanzas.
The audience's tolerance diminished during constant clinching, with Parker slipping to the canvas several times during close exchanges.
Parker ended stronger, but many rounds were difficult to score. Nevertheless, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just one round.
Buatsi progresses, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Tribute Fuels Local Rivalry Contest
In the supporting bout, Lyndon Arthur overcame Rea by close verdict in a excellent local rivalry fight to secure the European light heavyweight title.
The stadium fell silent for a profoundly moving homage ahead of the contest as ten bells rang in remembrance of local hero Ricky Hatton, who succumbed at age 46 in last month.
It was the first major televised fight night in the urban center since the passing of Hatton. As the final chime ended, the stillness gave way to a chant of "there's only one Hatton".
The action inside the ropes equaled the feeling. Arthur was sent to the canvas by a powerful punch squarely on the chin in the early round, rising shakily to his position on the eight count.
But the red-colored equipment of the fighter soon came forward again. On the opposite side, Rea - in rival City blue - was marked up soon, noticeable damage forming under his orbital area as the duo exchanged hard punches.
Rea, 27, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the intensity rarely dropped over 12 rounds.
The victor barely secured the verdict with judges scorecards of 114-114, close margin and 115-112, but both fighters were determined and full of heart, exactly what their inspiration the legend stood for.
