British light heavyweight Joshua Buatsi made a winning yet disappointing start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a close ten-round fight at the Manchester venue.
But the challenger landed significant shots, connecting with powerful clean blows in a contest where the approaches never quite clicked, with excessive holding spoiling the action.
The scoring judges scored it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my best display but what do champions do? They succeed," the victor said in the arena as he was faced negative reactions from the crowd.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in February - to secure a 20th career victory.
Various analysts believed Parker, thirty-one, should have been awarded the decision but the British boxer experienced the second defeat of his 28-match professional journey.
The Champion Secures Triumph in Uneventful Affair
His career development has slowed in recent times through promotional disputes and long spells of inactivity.
Earlier forecast as the future of the British fight scene, he now stands trying to realize his potential - perhaps in what appears to be a last chance - under fresh management.
He opened tentatively, with neither fighter prepared to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second fight at light heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for the favorite, who utilized a tight guard to barely take the more effective periods.
The audience's tolerance decreased amid frequent holding, with Parker slipping to the canvas various instances during grappling.
Parker closed well, but several stanzas were difficult to score. However, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just one round.
The victor advances, but with the presence of hard hitter Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Tribute Fuels Manchester Derby Fight
In the co-main event, Lyndon Arthur beat Rea by split-decision in a outstanding Manchester vs Manchester contest to win the continental belt.
The venue fell silent for a profoundly moving memorial ahead of the contest as 10 bells sounded in honor of Manchester's own Hatton, who succumbed aged 46 in September.
It was the premier televised boxing event in the location since Hatton's death. As the final chime disappeared, the stillness gave way to a chant of "there's just one Ricky Hatton".
The competition during the bout matched the feeling. Arthur was floored by a left hook flush on the face in the early round, standing uncertainly to his footing on the referee's count.
But the traditional red shorts of Arthur soon advanced again. Across the ring, Rea - in opposing blue - was injured quickly, heavy swelling forming under his orbital area as the pair landed powerful shots.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the pace seldom decreased over the full distance.
The victor barely secured the decision with scoring tallies of even score, narrow lead and 115-112, but the two athletes were persistent and full of heart, everything their idol the legend represented.
