UK light heavyweight Joshua Buatsi secured a victory yet disappointing debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a tight ten-round fight at the boxing arena.
But Parker landed significant shots, landing some telling individual punches in a match where the techniques didn't complement each other, with frequent clinching marring the contest.
Two judges scored it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my finest display but how do champions respond? They prevail," the victor commented in the ringside as he was faced disapproval from the crowd.
The Londoner bounced back after his maiden career setback - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Several boxing experts felt Parker, 31, should have been awarded the win but the Derbyshire fighter suffered the second setback of his 28-fight professional journey.
Buatsi Secures Victory in Slow Encounter
His career development has plateaued in recent years through management issues and extended periods of inactivity.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors prepared to take risks in a calculated opening few rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was participating in his second bout at the higher weight class.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to narrowly win the cleaner periods.
The spectators' acceptance diminished amid constant clinching, with Parker falling to the floor various instances during clinches.
Parker closed well, but several stanzas were challenging to assess. Although, former world champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, giving Buatsi just only one stanza.
Buatsi advances, but with the likes of power puncher David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Hatton Tribute Fuels Manchester Derby Battle
In the co-main event, Lyndon Arthur beat Rea by divided decision in a outstanding all-Manchester contest to secure the European light-heavyweight belt.
The stadium became quiet for a deeply poignant memorial ahead of the contest as 10 bells rang in memory of local hero Hatton, who succumbed at age 46 in recently.
It was the first major televised fight night in the location since Hatton's passing. As the final chime disappeared, the stillness transitioned to a chant of "there's only one Ricky Hatton".
The action within the ring matched the emotion. The eventual winner was floored by a powerful punch directly to the jaw in the initial stage, standing uncertainly to his feet on the eight count.
But the traditional red equipment of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was marked up quickly, noticeable damage developing under his face as the duo traded punishing blows.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur edged the result with scoring tallies of draw, narrow lead and 115-112, but the two athletes were persistent and full of heart, exactly what their hero the legend stood for.
