British light-heavyweight Joshua Buatsi secured a victory yet disappointing beginning to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a close 10-round fight at the boxing arena.
But the challenger landed significant shots, delivering effective individual punches in a fight where the styles failed to mesh, with excessive holding marring the action.
Two judges had it 96-94 to the winner, with the third a even score.
"It wasn't my best showing but what defines champions? They win," Buatsi commented in the ring as he was met by disapproval from the spectators.
The British fighter got back to winning after his initial pro loss - an thrilling contest with his previous opponent in last winter - to secure a 20th career victory.
Various analysts felt Parker, 31, deserved the win but the Derbyshire fighter endured the second loss of his 28-bout career.
The Champion Earns Win in Slow Affair
Buatsi's progress has plateaued in the past few years through contract problems and considerable time of inactivity.
Previously predicted as the future of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what feels like a last chance - under recent support.
He opened tentatively, with neither fighter willing to take risks in a cagey initial rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was competing in only his second bout at light heavyweight.
By the middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who operated with a tight guard to barely take the cleaner stanzas.
The crowd's patience decreased amid constant clinching, with Parker falling to the floor multiple occasions during grappling.
The underdog closed well, but multiple periods were hard to judge. Although, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the likes of heavy hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Inspires All-Manchester Battle
In the supporting bout, Lyndon Arthur overcame Rea by close verdict in a terrific Manchester vs Manchester fight to secure the European light heavyweight title.
The stadium fell silent for a profoundly moving homage ahead of the contest as 10 bells sounded in memory of the city's native Hatton, who succumbed at age 46 in last month.
It was the first major televised fight night in the location since Hatton's death. As the concluding sound ended, the stillness gave way to a chant of "only one Ricky Hatton".
The competition inside the ropes equaled the sentiment. The eventual winner was knocked down by a significant blow flush on the face in the early round, getting up unsteadily to his position on the eight count.
But the red-colored equipment of the fighter soon advanced again. From the other corner, Rea - in competitive blue - was injured quickly, noticeable damage developing under his eye as the fighters exchanged powerful shots.
The challenger, 27, landed power punches but Arthur, thirty-four, responded in kind with effective uppercuts as the pace rarely dropped over twelve rounds.
The victor narrowly won the decision with official scores of draw, narrow lead and wider gap, but each competitor were determined and full of heart, exactly what their inspiration Hatton embodied.
