UK light heavyweight Joshua Buatsi earned a win yet unimpressive debut to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a close 10-round fight at the boxing arena.
But Parker had his moments, connecting with powerful individual punches in a fight where the styles didn't complement each other, with frequent clinching marring the contest.
Two judges judged it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best showing but what do champions do? They succeed," Buatsi said in the arena as he was faced boos from the crowd.
The Londoner returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a twentieth professional win.
Some ringside observers thought the challenger, 31, should have been awarded the win but the British boxer suffered the second loss of his 28-bout professional journey.
The Champion Secures Triumph in Slow Affair
Buatsi's progress has slowed in the past few years through contract problems and considerable time of lack of fights.
Previously predicted as the next star of the British fight scene, he now stands trying to achieve expectations - possibly in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers prepared to be aggressive in a cagey early stages.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury during the bout, was making just his second appearance at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for Buatsi, who operated with a tight guard to edge the better stanzas.
The audience's tolerance wore thin during regular grabbing, with the challenger slipping to the canvas various instances during close exchanges.
The underdog ended stronger, but many rounds were difficult to score. Nevertheless, former world champion the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi progresses, but with the presence of heavy hitter Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Hatton Memorial Fuels Manchester Derby Fight
In the secondary fight, Arthur defeated Rea by close verdict in a excellent Manchester vs Manchester fight to win the continental title.
The venue fell silent for a extremely emotional homage ahead of the contest as ten bells rang in honor of the city's native Hatton, who succumbed aged 46 in recently.
It was the premier televised match card in the urban center since Hatton's death. As the concluding sound ended, the quiet gave way to a roar of "there's just one Ricky Hatton".
The competition within the ring reflected the feeling. Arthur was floored by a powerful punch squarely on the chin in the early round, standing uncertainly to his footing on the count of eight.
But the traditional red shorts of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was injured early, heavy swelling developing under his orbital area as the fighters exchanged punishing blows.
The challenger, twenty-seven, landed power punches but the champion, 34, fought fire with fire with stinging uppercuts as the pace rarely dropped over 12 rounds.
The victor narrowly won the decision with scoring tallies of 114-114, narrow lead and 115-112, but the two athletes were relentless and full of heart, all the qualities their hero Hatton represented.
