UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a close 10-round bout at the boxing arena.
But the challenger had his moments, landing some telling single shots in a fight where the approaches didn't complement each other, with frequent clinching spoiling the action.
The officials judged it 96-94 to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my best performance but what do champions do? They succeed," Buatsi said in the ringside as he was met by disapproval from the audience.
The British fighter got back to winning after his maiden career setback - an epic battle with his previous opponent in February - to earn a 20th career victory.
Some ringside observers believed the challenger, 31, should have been awarded the decision but the Derbyshire fighter experienced the second loss of his 28-bout professional journey.
Buatsi Claims Victory in Dull Match
Buatsi's progress has stalled in recent times through contract problems and long spells of inactivity.
Once tipped as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under recent support.
He opened tentatively, with the two boxers ready to be aggressive in a calculated initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand in the contest, was participating in his second appearance at the higher weight class.
By the halfway stage, the underdog had kept it close, causing problems for the favorite, who utilized a protective stance to barely take the more effective periods.
The audience's tolerance wore thin during constant clinching, with Parker slipping to the canvas multiple occasions during grappling.
Parker closed well, but multiple periods were hard to judge. Nevertheless, former world champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, giving Buatsi just only one stanza.
Buatsi advances, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Homage Fuels All-Manchester Fight
In the co-main event, Lyndon Arthur beat Rea by close verdict in a terrific local rivalry contest to secure the continental championship.
The arena fell silent for a profoundly moving homage ahead of the contest as a bell ceremony sounded in honor of the city's native Ricky Hatton, who succumbed at age 46 in last month.
It was the premier televised match card in the city since the passing of Hatton. As the last bell faded, the quiet became a chant of "there's just one Ricky Hatton".
The competition inside the ropes matched the feeling. Arthur was floored by a left hook flush on the chin in the second round, getting up unsteadily to his feet on the eight count.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, Rea - in opposing blue - was damaged early, significant bruising developing under his face as the duo landed punishing blows.
The challenger, 27, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the intensity hardly diminished over twelve rounds.
Arthur narrowly won the result with official scores of even score, close margin and wider gap, but both fighters were relentless and full of heart, exactly what their idol Hatton stood for.
