British light heavyweight Joshua Buatsi earned a win yet disappointing start to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a split decision in a close ten-round contest at the boxing arena.
But Parker had his moments, connecting with powerful individual punches in a contest where the approaches failed to mesh, with excessive holding disrupting the fight.
The officials judged it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my finest performance but what defines champions? They win," Buatsi said in the arena as he was faced boos from the crowd.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a twentieth professional win.
Several boxing experts felt Parker, 31, merited the win but the British boxer experienced the second setback of his 28-fight professional journey.
The Champion Earns Triumph in Dull Match
His career development has stalled in recent times through management issues and long spells of inactivity.
Once tipped as the future of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a last chance - under recent support.
He opened tentatively, with both competitors ready to take risks in a cagey early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury in the contest, was making just his second fight at light-heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for the favorite, who worked behind a tight guard to barely take the cleaner periods.
The audience's tolerance decreased amid constant clinching, with Parker going down multiple occasions during grappling.
Parker finished the stronger, but several stanzas were hard to judge. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor progresses, but with the likes of hard hitter Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Homage Motivates All-Manchester Fight
In the secondary fight, Lyndon Arthur defeated Rea by close verdict in a excellent local rivalry battle to capture the European light heavyweight belt.
The stadium became quiet for a deeply poignant homage prior to the bout as 10 bells tolled in honor of Manchester's own Ricky Hatton, who died at age 46 in recently.
It was the first major televised fight night in the urban center since Hatton's death. As the final chime disappeared, the silence transitioned to a shout of "there's just one Ricky Hatton".
The action within the ring matched the sentiment. Arthur was floored by a left hook directly to the jaw in the early round, rising shakily to his feet on the count of eight.
But the traditional red attire of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was marked up quickly, significant bruising appearing under his face as the pair exchanged powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, 34, responded in kind with powerful upward punches as the action rarely dropped over 12 rounds.
Arthur barely secured the result with scoring tallies of 114-114, narrow lead and clearer margin, but the two athletes were determined and brave, exactly what their idol Hatton stood for.
