British light heavyweight Joshua Buatsi secured a victory yet unimpressive debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a competitive 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a contest where the approaches never quite clicked, with constant grabbing spoiling the contest.
Two judges scored it 96-94 for to the winner, with the third a even score.
"It wasn't my finest display but how do champions respond? They succeed," the victor stated in the ringside as he was greeted with disapproval from the audience.
The Londoner returned to winning ways after his first professional defeat - an epic battle with his previous opponent in February - to secure a 20th pro success.
Several boxing experts thought the challenger, thirty-one, deserved the win but the contender suffered the second setback of his 28-bout professional journey.
The Champion Claims Victory in Uneventful Encounter
His career development has slowed in recent years through promotional disputes and long spells of inactivity.
Previously predicted as the next star of British boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a final roll of the dice - under fresh management.
He started carefully, with the two boxers willing to overcommit in a tentative initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was making just his second fight at light heavyweight.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who utilized a solid defense to barely take the better stanzas.
The crowd's patience wore thin during frequent holding, with Parker falling to the floor various instances during close exchanges.
Parker closed well, but multiple periods were difficult to score. Nevertheless, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor progresses, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Homage Motivates Local Rivalry Fight
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a excellent all-Manchester fight to secure the continental title.
The stadium grew hushed for a profoundly moving homage before the fight as ten bells tolled in memory of the city's native Ricky Hatton, who succumbed 46 years old in September.
It was the premier televised boxing event in the location since the passing of Hatton. As the last bell disappeared, the silence became a shout of "there's just one Hatton".
The competition inside the ropes equaled the feeling. Arthur was knocked down by a significant blow directly to the jaw in the initial stage, standing uncertainly to his feet on the referee's count.
But the red-colored equipment of the fighter soon advanced again. From the other corner, the opponent - in rival City blue - was damaged soon, noticeable damage forming under his orbital area as the pair exchanged hard punches.
The challenger, twenty-seven, landed power punches but Arthur, 34, matched the intensity with stinging uppercuts as the action hardly diminished over twelve rounds.
Arthur barely secured the result with judges scorecards of even score, narrow lead and wider gap, but the two athletes were relentless and courageous, all the qualities their hero the legend embodied.
