UK light-heavyweight Joshua Buatsi earned a win yet unimpressive beginning to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a tight 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, landing some telling clean blows in a match where the approaches didn't complement each other, with excessive holding marring the fight.
The officials had it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my greatest showing but how do champions respond? They prevail," the victor stated in the ring as he was met by boos from the crowd.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a 20th career victory.
Several boxing experts believed Parker, 31, deserved the victory but the British boxer suffered the second setback of his 28-match career.
Buatsi Claims Victory in Slow Affair
His career development has stalled in the past few years through contract problems and extended periods of inactivity.
Previously predicted as the future of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers ready to take risks in a tentative initial rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after suffering a hand injury during the bout, was participating in his second appearance at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who operated with a solid defense to edge the more effective rounds.
The crowd's patience decreased amid constant clinching, with Parker slipping to the canvas multiple occasions during grappling.
The underdog finished the stronger, but multiple periods were hard to judge. However, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the presence of heavy hitter Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - based on this performance - still appears challenging.
Hatton Memorial Fuels All-Manchester Contest
In the co-main event, Lyndon Arthur beat Rea by divided decision in a excellent Manchester vs Manchester contest to win the European light-heavyweight championship.
The stadium grew hushed for a extremely emotional memorial ahead of the contest as ten bells sounded in memory of the city's native Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the concluding sound ended, the stillness gave way to a chant of "there's just one Ricky Hatton".
The competition within the ring reflected the emotion. Arthur was sent to the canvas by a powerful punch squarely on the chin in the early round, standing uncertainly to his footing on the referee's count.
But the traditional red attire of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was damaged early, noticeable damage developing under his face as the fighters traded punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with powerful upward punches as the intensity seldom decreased over twelve rounds.
Arthur barely secured the result with official scores of 114-114, 115-113 and clearer margin, but the two athletes were persistent and full of heart, exactly what their inspiration Hatton stood for.
