UK light heavyweight Joshua Buatsi secured a victory yet disappointing debut to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a tight 10-round fight at the Manchester venue.
But Parker found success, delivering effective individual punches in a contest where the styles didn't complement each other, with constant grabbing disrupting the action.
The officials judged it 96-94 for to the winner, with the third a 95-95.
"It wasn't my finest showing but what do champions do? They prevail," Buatsi said in the ring as he was greeted with boos from the audience.
The British fighter got back to winning after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a 20th career victory.
Some ringside observers believed Parker, thirty-one, merited the win but the Derbyshire fighter endured the second defeat of his 28-bout professional journey.
Buatsi Secures Triumph in Slow Encounter
The fighter's advancement has plateaued in recent years through contract problems and extended periods of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under recent support.
He began cautiously, with both competitors willing to overcommit in a calculated opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was making just his second appearance at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for Buatsi, who utilized a protective stance to edge the cleaner stanzas.
The crowd's patience wore thin during constant clinching, with Parker falling to the floor various instances during grappling.
Parker finished the stronger, but many rounds were challenging to assess. However, former world champion the boxing analyst - working with the broadcast network - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - on this evidence - still seems difficult.
Hatton Tribute Motivates All-Manchester Fight
In the supporting bout, Arthur beat Rea by split-decision in a excellent all-Manchester contest to capture the continental title.
The arena became quiet for a deeply poignant memorial prior to the bout as a bell ceremony rang in remembrance of local hero Ricky Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the final chime disappeared, the silence became a shout of "there's only one Hatton".
The competition inside the ropes matched the feeling. Arthur was floored by a left hook directly to the chin in the initial stage, rising shakily to his feet on the eight count.
But the traditional red equipment of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was injured early, significant bruising developing under his orbital area as the pair traded hard punches.
Rea, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with effective uppercuts as the action seldom decreased over the full distance.
The victor narrowly won the result with scoring tallies of draw, narrow lead and clearer margin, but both fighters were determined and brave, everything their hero the legend embodied.
