UK light-heavyweight Joshua Buatsi earned a win yet unimpressive beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a split decision in a close 10-round bout at the Manchester venue.
But Parker found success, landing some telling single shots in a fight where the styles failed to mesh, with frequent clinching marring the action.
Two judges scored it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my best performance but how do champions respond? They win," the victor commented in the arena as he was greeted with boos from the crowd.
The British fighter bounced back after his first professional defeat - an memorable fight with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts felt the challenger, thirty-one, merited the decision but the Derbyshire fighter experienced the second setback of his 28-bout professional journey.
Buatsi Earns Win in Slow Encounter
His career development has plateaued in the past few years through management issues and considerable time of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to achieve expectations - maybe in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with both competitors ready to overcommit in a calculated initial rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was participating in his second bout at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who worked behind a tight guard to barely take the better periods.
The spectators' acceptance decreased during frequent holding, with the challenger falling to the floor various instances during grappling.
The underdog closed well, but multiple periods were difficult to score. However, former world champion Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the likes of power puncher Benavidez, and Russian champion Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a excellent Manchester vs Manchester battle to secure the European light heavyweight belt.
The venue became quiet for a profoundly moving homage before the fight as a bell ceremony tolled in memory of the city's native Hatton, who succumbed 46 years old in recently.
It was the premier televised boxing event in the urban center since Hatton's death. As the last bell disappeared, the silence became a roar of "only one Ricky Hatton".
The action inside the ropes equaled the sentiment. Arthur was floored by a left hook flush on the face in the second round, rising shakily to his footing on the eight count.
But the Manchester United-red equipment of the fighter soon came forward again. From the other corner, the opponent - in opposing blue - was injured early, noticeable damage appearing under his eye as the pair exchanged powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with powerful upward punches as the intensity rarely dropped over 12 rounds.
The victor barely secured the decision with official scores of draw, narrow lead and 115-112, but each competitor were determined and courageous, everything their hero the legend represented.
