UK light heavyweight Joshua Buatsi secured a victory yet disappointing debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a competitive ten-round bout at the Manchester venue.
But the challenger found success, delivering effective clean blows in a fight where the techniques didn't complement each other, with excessive holding disrupting the fight.
The scoring judges had it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest performance but how do champions respond? They prevail," the victor said in the ringside as he was faced boos from the spectators.
The Londoner bounced back after his first professional defeat - an epic battle with Callum Smith in February - to earn a 20th pro success.
Various analysts believed Parker, 31, should have been awarded the decision but the contender suffered the second loss of his 28-fight professional journey.
The Champion Secures Triumph in Uneventful Match
Buatsi's progress has plateaued in the past few years through contract problems and considerable time of lack of fights.
Earlier forecast as the next star of British boxing, he now finds himself trying to achieve expectations - maybe in what appears to be a last chance - under new promotional backing.
He started carefully, with the two boxers prepared to be aggressive in a tentative opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was making just his second appearance at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, causing problems for Buatsi, who operated with a solid defense to edge the cleaner periods.
The audience's tolerance decreased because of regular grabbing, with the challenger falling to the floor multiple occasions during close exchanges.
Parker ended stronger, but many rounds were challenging to assess. Nevertheless, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor advances, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Fuels Manchester Derby Contest
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a outstanding Manchester vs Manchester contest to capture the European light-heavyweight belt.
The stadium became quiet for a deeply poignant tribute before the fight as ten bells sounded in remembrance of local hero Ricky Hatton, who died aged 46 in last month.
It was the first major televised match card in the urban center since Hatton's passing. As the final chime ended, the stillness became a chant of "there's only one Ricky Hatton".
The fighting within the ring matched the feeling. Arthur was sent to the canvas by a significant blow directly to the jaw in the early round, standing uncertainly to his feet on the eight count.
But the traditional red shorts of Arthur soon pressed ahead again. Across the ring, Rea - in competitive blue - was damaged soon, significant bruising forming under his orbital area as the duo landed powerful shots.
Rea, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with powerful upward punches as the pace rarely dropped over the full distance.
Arthur narrowly won the decision with scoring tallies of 114-114, close margin and wider gap, but each competitor were determined and courageous, exactly what their hero the legend stood for.
