British light heavyweight Joshua Buatsi made a winning yet disappointing beginning to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a tight ten-round contest at the boxing arena.
But Parker landed significant shots, delivering effective individual punches in a match where the techniques failed to mesh, with frequent clinching marring the action.
The scoring judges scored it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my best performance but what defines champions? They prevail," the victor stated in the ringside as he was met by negative reactions from the audience.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a 20th career victory.
Several boxing experts believed Parker, thirty-one, deserved the victory but the contender suffered the second loss of his 28-match professional journey.
Buatsi Claims Triumph in Slow Encounter
The fighter's advancement has stalled in the past few years through promotional disputes and long spells of inactivity.
Earlier forecast as the future of the British fight scene, he now stands trying to realize his potential - perhaps in what feels like a last chance - under fresh management.
He started carefully, with both competitors willing to take risks in a calculated initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second appearance at light-heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who operated with a tight guard to edge the more effective periods.
The crowd's patience diminished during regular grabbing, with the challenger slipping to the canvas multiple occasions during close exchanges.
The underdog finished the stronger, but multiple periods were hard to judge. However, ex-champion the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
The victor advances, but with the presence of hard hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Memorial Fuels All-Manchester Contest
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a terrific local rivalry battle to capture the European light heavyweight title.
The venue fell silent for a profoundly moving tribute before the fight as ten bells rang in memory of the city's native Hatton, who passed away aged 46 in recently.
It was the first major televised match card in the location since the passing of Hatton. As the last bell faded, the silence gave way to a shout of "only one Hatton".
The competition during the bout equaled the emotion. Arthur was sent to the canvas by a significant blow squarely on the chin in the early round, rising shakily to his footing on the count of eight.
But the red-colored shorts of the fighter soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was damaged early, heavy swelling appearing under his eye as the duo traded punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with stinging uppercuts as the action rarely dropped over twelve rounds.
The victor barely secured the result with scoring tallies of 114-114, narrow lead and 115-112, but the two athletes were relentless and full of heart, exactly what their idol the legend embodied.
