UK light heavyweight Joshua Buatsi secured a victory yet underwhelming start to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a tight ten-round contest at the Co-op Live Arena.
But Parker had his moments, connecting with powerful clean blows in a match where the techniques didn't complement each other, with constant grabbing marring the contest.
Two judges had it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest performance but what do champions do? They win," Buatsi said in the arena as he was faced negative reactions from the audience.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to secure a 20th pro success.
Several boxing experts believed Parker, 31, merited the win but the British boxer endured the second defeat of his 28-fight professional journey.
The Champion Claims Win in Dull Affair
The fighter's advancement has stalled in the past few years through management issues and considerable time of inactivity.
Once tipped as the future of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with both competitors willing to overcommit in a calculated early stages.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand in the contest, was making just his second bout at light heavyweight.
By the middle rounds, the underdog had kept it close, making life awkward for the favorite, who operated with a protective stance to narrowly win the cleaner stanzas.
The audience's tolerance diminished because of frequent holding, with the challenger falling to the floor multiple occasions during close exchanges.
Parker finished the stronger, but many rounds were challenging to assess. Nevertheless, former world champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
Buatsi advances, but with the presence of heavy hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Memorial Motivates All-Manchester Contest
In the co-main event, Arthur beat Bradley Rea by close verdict in a terrific local rivalry fight to secure the European light heavyweight belt.
The arena 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 last month.
It was the premier televised fight night in the city since Hatton's passing. As the final chime faded, the silence gave way to a roar of "there's just one Ricky Hatton".
The action within the ring matched the emotion. The eventual winner was knocked down by a powerful punch directly to the chin in the early round, rising shakily to his position on the count of eight.
But the Manchester United-red attire of Arthur soon pressed ahead again. From the other corner, Rea - in opposing blue - was injured soon, noticeable damage appearing under his face as the pair landed powerful shots.
The challenger, 27, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the action hardly diminished over twelve rounds.
Arthur barely secured the decision with scoring tallies of even score, close margin and 115-112, but both fighters were persistent and brave, all the qualities their idol the legend embodied.
