British light-heavyweight Joshua Buatsi earned a win yet disappointing start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a tight ten-round bout at the Manchester venue.
But Parker landed significant shots, connecting with powerful single shots in a fight where the techniques failed to mesh, with excessive holding marring the action.
Two judges had it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my finest showing but what defines champions? They prevail," Buatsi commented in the ring as he was faced disapproval from the audience.
The Londoner returned to winning ways after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Several boxing experts felt Parker, thirty-one, deserved the win but the British boxer experienced the second setback of his 28-match career.
The Champion Secures Win in Dull Match
His career development has stalled in the past few years through management issues and extended periods of lack of fights.
Once tipped as the next star of British boxing, he now is positioned trying to realize his potential - perhaps in what seems like a ultimate opportunity - under recent support.
He began cautiously, with the two boxers willing to overcommit in a calculated initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was making just his second bout at light heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for Buatsi, who operated with a tight guard to narrowly win the cleaner periods.
The spectators' acceptance diminished during regular grabbing, with Parker going down various instances during grappling.
The underdog ended stronger, but several stanzas were hard to judge. Nevertheless, previous titleholder the boxing analyst - working with the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just one round.
The victor moves on, but with the likes of hard hitter Benavidez, and Russian champion Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Hatton Tribute Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a outstanding Manchester vs Manchester contest to capture the European light-heavyweight championship.
The venue fell silent for a profoundly moving homage before the fight as ten bells rang in honor of local hero Ricky Hatton, who passed away at age 46 in September.
It was the premier televised fight night in the city since the passing of Hatton. As the concluding sound ended, the stillness became a chant of "only one Ricky Hatton".
The competition within the ring matched the feeling. The eventual winner was sent to the canvas by a powerful punch flush on the face in the early round, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red equipment of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was marked up soon, noticeable damage forming under his orbital area as the duo landed hard punches.
The challenger, 27, delivered effective strikes but the champion, thirty-four, fought fire with fire with stinging uppercuts as the pace hardly diminished over the full distance.
Arthur barely secured the verdict with judges scorecards of even score, close margin and wider gap, but each competitor were persistent and full of heart, everything their inspiration Hatton represented.
