British light-heavyweight Joshua Buatsi made a winning yet unimpressive beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a close 10-round contest at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful individual punches in a match where the techniques failed to mesh, with excessive holding spoiling the fight.
The officials had it 96-94 for to the winner, with the third a 95-95.
"It wasn't my finest showing but what defines champions? They succeed," the victor stated in the ring as he was greeted with boos from the audience.
The Londoner bounced back after his first professional defeat - an memorable fight with Callum Smith in February - to earn a 20th pro success.
Some ringside observers believed Parker, thirty-one, merited the win but the British boxer experienced the second defeat of his 28-match professional journey.
Buatsi Earns Triumph in Dull Affair
His career development has slowed in recent years through promotional disputes and considerable time of lack of fights.
Earlier forecast as the future of UK boxing, he now stands trying to achieve expectations - maybe in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers ready to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was participating in his second fight at the higher weight class.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who utilized a solid defense to edge the better rounds.
The crowd's patience wore thin amid frequent holding, with the challenger slipping to the canvas several times during close exchanges.
Parker finished the stronger, but several stanzas were challenging to assess. However, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi progresses, but with the likes of heavy hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Hatton Tribute Fuels Local Rivalry Battle
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a outstanding all-Manchester fight to win the European light-heavyweight championship.
The arena became quiet for a deeply poignant memorial before the fight as 10 bells tolled in remembrance of Manchester's own Ricky Hatton, who passed away aged 46 in September.
It was the premier televised match card in the location since the passing of Hatton. As the final chime faded, the quiet gave way to a chant of "only one Hatton".
The fighting during the bout reflected the sentiment. Arthur was sent to the canvas by a powerful punch directly to the jaw in the second round, getting up unsteadily to his footing on the referee's count.
But the red-colored shorts of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was injured early, significant bruising forming under his orbital area as the fighters traded hard punches.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with stinging uppercuts as the intensity seldom decreased over twelve rounds.
The victor barely secured the result with scoring tallies of draw, 115-113 and clearer margin, but each competitor were determined and brave, exactly what their hero Hatton embodied.
