British light heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a competitive 10-round fight at the Manchester venue.
But Parker had his moments, landing some telling clean blows in a fight where the techniques failed to mesh, with excessive holding marring the fight.
Two judges scored it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my finest showing but what defines champions? They win," Buatsi said in the ring as he was met by boos from the spectators.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with his previous opponent in last winter - to earn a 20th pro success.
Some ringside observers thought Parker, thirty-one, merited the victory but the British boxer endured the second loss of his 28-bout professional journey.
Buatsi Secures Win in Slow Affair
His career development has stalled in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the future of the British fight scene, he now is positioned trying to achieve expectations - possibly in what feels like a final roll of the dice - under recent support.
He opened tentatively, with both competitors prepared to take risks in a cagey opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after suffering a hand injury during the bout, was making just his second fight at light heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for the favorite, who utilized a protective stance to edge the more effective periods.
The crowd's patience diminished amid constant clinching, with the challenger slipping to the canvas various instances during clinches.
The underdog closed well, but multiple periods were challenging to assess. However, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the presence of power puncher Benavidez, and Russian champion Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Hatton Tribute Fuels All-Manchester Fight
In the secondary fight, Lyndon Arthur overcame Rea by split-decision in a outstanding local rivalry battle to secure the European light heavyweight title.
The arena grew hushed for a extremely emotional homage ahead of the contest as ten bells rang in remembrance of Manchester's own Hatton, who succumbed at age 46 in recently.
It was the first major televised match card in the city since Hatton's death. As the concluding sound faded, the silence transitioned to a chant of "there's just one Ricky Hatton".
The fighting during the bout reflected the sentiment. The eventual winner was knocked down by a significant blow squarely on the chin in the second round, rising shakily to his footing on the count of eight.
But the red-colored attire of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was injured quickly, noticeable damage forming under his eye as the duo traded punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with powerful upward punches as the pace hardly diminished over twelve rounds.
The victor narrowly won the verdict with scoring tallies of draw, 115-113 and clearer margin, but both fighters were persistent and full of heart, exactly what their hero Hatton represented.
