British light-heavyweight Joshua Buatsi made a winning yet underwhelming start to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a split decision in a competitive ten-round fight at the boxing arena.
But the challenger landed significant shots, delivering effective single shots in a contest where the approaches failed to mesh, with excessive holding marring the fight.
The scoring judges had it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my finest performance but what do champions do? They succeed," Buatsi stated in the ringside as he was greeted with negative reactions from the spectators.
The Londoner got back to winning after his maiden career setback - an memorable fight with his previous opponent in last winter - to earn a 20th pro success.
Various analysts felt Parker, thirty-one, merited the win but the British boxer endured the second setback of his 28-fight career.
The Champion Claims Triumph in Dull Affair
Buatsi's progress has stalled in recent years through management issues and long spells of inactivity.
Previously predicted as the future of the British fight scene, he now stands trying to fulfil that early promise - maybe in what seems like a last chance - under fresh management.
He opened tentatively, with neither fighter willing to overcommit in a cagey opening few rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was participating in his second appearance at light heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for the favorite, who operated with a protective stance to barely take the better periods.
The spectators' acceptance decreased amid regular grabbing, with Parker going down several times during grappling.
The underdog ended stronger, but several stanzas were difficult to score. However, former world champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, awarding Buatsi just one round.
The victor advances, but with the caliber of power puncher Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Hatton Tribute Motivates All-Manchester Contest
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a outstanding all-Manchester battle to capture the European light-heavyweight championship.
The stadium fell silent for a deeply poignant memorial ahead of the contest as 10 bells tolled in memory of local hero Ricky Hatton, who died at age 46 in last month.
It was the premier televised boxing event in the urban center since Hatton's death. As the final chime ended, the stillness gave way to a shout of "there's only one Hatton".
The competition inside the ropes equaled the feeling. Arthur was knocked down by a powerful punch directly to the jaw in the second round, standing uncertainly to his position on the eight count.
But the red-colored equipment of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was damaged quickly, significant bruising developing under his eye as the pair traded powerful shots.
The challenger, 27, landed power punches but Arthur, 34, fought fire with fire with stinging uppercuts as the intensity rarely dropped over 12 rounds.
Arthur edged the verdict with scoring tallies of even score, close margin and wider gap, but each competitor were relentless and brave, all the qualities their idol Hatton embodied.
