UK light heavyweight Joshua Buatsi earned a win yet disappointing beginning to his career under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a split decision in a close 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, landing some telling clean blows in a contest where the approaches never quite clicked, with excessive holding spoiling the fight.
The scoring judges had it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my best performance but how do champions respond? They win," the victor said in the arena as he was greeted with negative reactions from the crowd.
The Londoner returned to winning ways after his maiden career setback - an epic battle with his previous opponent in February - to claim a 20th pro success.
Various analysts thought Parker, 31, deserved the victory but the contender experienced the second setback of his 28-match career.
Buatsi Claims Win in Slow Affair
The fighter's advancement has slowed in the past few years through promotional disputes and considerable time of lack of fights.
Once tipped as the rising talent of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under fresh management.
He began cautiously, with the two boxers ready to overcommit in a calculated opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second bout at light heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for Buatsi, who worked behind a solid defense to barely take the cleaner rounds.
The audience's tolerance wore thin because of constant clinching, with the challenger falling to the floor multiple occasions during close exchanges.
The underdog closed well, but many rounds were challenging to assess. Although, previous titleholder the boxing analyst - working with the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the presence of heavy hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still looks steep.
Hatton Tribute Motivates All-Manchester Contest
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a terrific Manchester vs Manchester battle to secure the European light heavyweight belt.
The arena became quiet for a profoundly moving tribute before the fight as ten bells tolled in honor of local hero Ricky Hatton, who passed away 46 years old in last month.
It was the premier televised fight night in the city since Hatton's death. As the last bell faded, the silence gave way to a roar of "there's only one Hatton".
The fighting during the bout equaled the feeling. The eventual winner was sent to the canvas by a powerful punch squarely on the chin in the early round, standing uncertainly to his footing on the count of eight.
But the red-colored attire of the fighter soon pressed ahead again. From the other corner, Rea - in rival City blue - was marked up soon, significant bruising appearing under his eye as the fighters traded powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with powerful upward punches as the action hardly diminished over 12 rounds.
Arthur barely secured the verdict with scoring tallies of draw, narrow lead and wider gap, but both fighters were relentless and brave, all the qualities their idol Hatton represented.
