UK light heavyweight Joshua Buatsi made a winning yet disappointing debut to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a split decision in a tight 10-round bout at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful clean blows in a contest where the approaches didn't complement each other, with frequent clinching spoiling the fight.
Two judges scored it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my finest showing but what defines champions? They prevail," Buatsi stated in the ringside as he was met by boos from the audience.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in February - to earn a 20th pro success.
Some ringside observers felt Parker, thirty-one, merited the victory but the contender experienced the second setback of his 28-fight career.
The Champion Claims Win in Uneventful Encounter
The fighter's advancement has stalled in recent times through management issues and considerable time of inactivity.
Once tipped as the rising talent of UK boxing, he now is positioned trying to realize his potential - perhaps in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with both competitors willing to be aggressive in a tentative opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after damaging his hand mid-fight, was participating in his second bout at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for Buatsi, who worked behind a solid defense to edge the better rounds.
The crowd's patience decreased during constant clinching, with the challenger falling to the floor multiple occasions during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. Nevertheless, ex-champion the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi progresses, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Homage Motivates Local Rivalry Battle
In the secondary fight, Arthur defeated Rea by close verdict in a terrific local rivalry fight to win the European light-heavyweight belt.
The arena became quiet for a extremely emotional homage prior to the bout as 10 bells tolled in remembrance of Manchester's own Hatton, who died aged 46 in last month.
It was the initial significant broadcast fight night in the location since Hatton's death. As the last bell ended, the silence transitioned to a shout of "only one Ricky Hatton".
The competition during the bout matched the sentiment. Arthur was sent to the canvas by a powerful punch flush on the chin in the second round, standing uncertainly to his footing on the referee's count.
But the red-colored attire of the fighter soon came forward again. From the other corner, Rea - in opposing blue - was marked up soon, noticeable damage developing under his eye as the fighters traded punishing blows.
Rea, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the intensity rarely dropped over twelve rounds.
Arthur barely secured the result with official scores of even score, narrow lead and 115-112, but each competitor were relentless and full of heart, everything their hero Hatton stood for.
