UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a tight 10-round fight at the Co-op Live Arena.
But the challenger found success, landing some telling clean blows in a match where the styles failed to mesh, with constant grabbing marring the contest.
The scoring judges had it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest showing but what do champions do? They win," Buatsi said in the ring as he was greeted with disapproval from the crowd.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in February - to secure a 20th career victory.
Several boxing experts thought the challenger, 31, deserved the decision but the British boxer endured the second setback of his 28-match career.
The Champion Claims Victory in Slow Encounter
The fighter's advancement has stalled in the past few years through management issues and considerable time of inactivity.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under recent support.
He began cautiously, with both competitors ready to be aggressive in a calculated initial rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second appearance at light-heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who worked behind a solid defense to narrowly win the better periods.
The audience's tolerance wore thin during regular grabbing, with the challenger slipping to the canvas several times during close exchanges.
Parker ended stronger, but several stanzas were challenging to assess. Although, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor moves on, but with the presence of power puncher Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Memorial Fuels Local Rivalry Battle
In the supporting bout, Lyndon Arthur overcame Rea by divided decision in a terrific all-Manchester fight to win the European light heavyweight belt.
The venue fell silent for a deeply poignant tribute prior to the bout as ten bells sounded in memory of Manchester's own Ricky Hatton, who died 46 years old in recently.
It was the premier televised fight night in the location since the passing of Hatton. As the last bell disappeared, the quiet transitioned to a chant of "there's just one Hatton".
The competition within the ring equaled the feeling. The eventual winner was knocked down by a left hook flush on the jaw in the second round, standing uncertainly to his position on the count of eight.
But the Manchester United-red equipment of Arthur soon pressed ahead again. Across the ring, Rea - in rival City blue - was marked up soon, noticeable damage appearing under his orbital area as the fighters exchanged powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with effective uppercuts as the pace seldom decreased over twelve rounds.
The victor narrowly won the decision with scoring tallies of draw, close margin and wider gap, but the two athletes were persistent and courageous, everything their inspiration Hatton embodied.
