British light heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a close 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful single shots in a fight where the styles never quite clicked, with constant grabbing marring the fight.
Two judges scored it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest showing but what do champions do? They prevail," the victor commented in the arena as he was greeted with boos from the spectators.
The Londoner bounced back after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Various analysts felt Parker, 31, deserved the decision but the contender endured the second defeat of his 28-fight professional journey.
The Champion Claims Victory in Slow Affair
Buatsi's progress has stalled in recent years through contract problems and considerable time of inactivity.
Earlier forecast as the rising talent of British boxing, he now stands trying to realize his potential - perhaps in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with neither fighter prepared to overcommit in a calculated opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second bout at light heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for Buatsi, who operated with a protective stance to barely take the cleaner rounds.
The crowd's patience wore thin amid constant clinching, with Parker falling to the floor several times during grappling.
The underdog ended stronger, but multiple periods were challenging to assess. Although, former world champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of power puncher Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Hatton Tribute Fuels All-Manchester Contest
In the secondary fight, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific Manchester vs Manchester fight to capture the European light heavyweight championship.
The stadium grew hushed for a deeply poignant homage before the fight as a bell ceremony tolled in honor of local hero Hatton, who died aged 46 in recently.
It was the initial significant broadcast fight night in the location since the passing of Hatton. As the final chime faded, the silence became a roar of "there's just one Hatton".
The fighting within the ring reflected the feeling. Arthur was knocked down by a significant blow directly to the chin in the early round, standing uncertainly to his feet on the count of eight.
But the traditional red attire of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was injured quickly, noticeable damage developing under his orbital area as the pair traded powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, responded in kind with stinging uppercuts as the pace hardly diminished over 12 rounds.
The victor barely secured the result with official scores of 114-114, close margin and clearer margin, but each competitor were relentless and courageous, exactly what their hero Hatton embodied.
