British light heavyweight Joshua Buatsi earned a win yet unimpressive beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a close 10-round contest at the Manchester venue.
But the challenger had his moments, landing some telling single shots in a contest where the techniques never quite clicked, with constant grabbing disrupting the action.
Two judges had it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my best performance but what do champions do? They succeed," the victor said in the ringside as he was met by boos from the spectators.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Various analysts thought Parker, thirty-one, should have been awarded the win but the Derbyshire fighter endured the second defeat of his 28-bout professional journey.
Buatsi Secures Victory in Dull Encounter
The fighter's advancement has stalled in recent years through promotional disputes and long spells of lack of fights.
Previously predicted as the rising talent of UK boxing, he now is positioned trying to realize his potential - perhaps in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter prepared to overcommit in a calculated opening few rounds.
The challenger, 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 halfway stage, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a protective stance to barely take the better stanzas.
The audience's tolerance diminished because of frequent holding, with the challenger slipping to the canvas several times during clinches.
Parker closed well, but multiple periods were challenging to assess. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, scoring for Buatsi just one round.
Buatsi advances, but with the likes of heavy hitter Benavidez, and Russian champion Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Inspires Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Rea by split-decision in a outstanding all-Manchester fight to win the continental belt.
The venue grew hushed for a deeply poignant tribute ahead of the contest as 10 bells rang in memory of local hero Ricky Hatton, who succumbed aged 46 in recently.
It was the first major televised match card in the city since Hatton's passing. As the concluding sound faded, the silence gave way to a chant of "there's only one Hatton".
The competition inside the ropes matched the emotion. The eventual winner was sent to the canvas by a left hook squarely on the chin in the early round, standing uncertainly to his feet on the count of eight.
But the traditional red equipment of the fighter soon advanced again. From the other corner, Rea - in opposing blue - was marked up quickly, heavy swelling forming under his orbital area as the fighters exchanged powerful shots.
Rea, 27, connected with strong blows but the champion, 34, matched the intensity with stinging uppercuts as the action seldom decreased over 12 rounds.
The victor narrowly won the decision with judges scorecards of even score, narrow lead and clearer margin, but each competitor were persistent and brave, all the qualities their hero Hatton represented.
