British light-heavyweight Joshua Buatsi made a winning yet unimpressive debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a competitive ten-round bout at the Co-op Live Arena.
But Parker had his moments, connecting with powerful clean blows in a match where the styles failed to mesh, with constant grabbing disrupting the fight.
The scoring judges scored it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my best performance but how do champions respond? They win," Buatsi stated in the ring as he was met by negative reactions from the spectators.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to secure a 20th pro success.
Various analysts believed Parker, 31, deserved the decision but the contender endured the second setback of his 28-fight professional journey.
Buatsi Secures Win in Dull Encounter
His career development has stalled in recent years through management issues and extended periods of lack of fights.
Once tipped as the rising talent of British boxing, he now finds himself trying to realize his potential - possibly in what feels like a final roll of the dice - under fresh management.
He began cautiously, with the two boxers willing to be aggressive in a calculated opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after breaking his hand mid-fight, was participating in his second appearance at the higher weight class.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a protective stance to barely take the better rounds.
The crowd's patience decreased during constant clinching, with the challenger slipping to the canvas multiple occasions during clinches.
Parker finished the stronger, but multiple periods were hard to judge. However, ex-champion the boxing analyst - working with Dazn - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor advances, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Hatton Memorial Inspires Manchester Derby Battle
In the secondary fight, Arthur defeated Rea by split-decision in a terrific Manchester vs Manchester fight to capture the continental title.
The venue became quiet for a extremely emotional homage before the fight as 10 bells sounded in honor of local hero Ricky Hatton, who died at age 46 in recently.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the concluding sound disappeared, the silence gave way to a chant of "there's just one Hatton".
The competition during the bout equaled the emotion. The eventual winner was floored by a left hook flush on the chin in the early round, standing uncertainly to his footing on the count of eight.
But the traditional red attire of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was damaged early, significant bruising forming under his orbital area as the pair exchanged punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the action hardly diminished over twelve rounds.
Arthur edged the result with official scores of 114-114, close margin and 115-112, but the two athletes were relentless and full of heart, everything their hero the legend stood for.
