British light-heavyweight Joshua Buatsi made a winning yet disappointing debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a close 10-round bout at the Co-op Live Arena.
But Parker found success, delivering effective individual punches in a fight where the styles didn't complement each other, with frequent clinching disrupting the action.
The officials judged it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best performance but what defines champions? They prevail," the victor stated in the ringside as he was greeted with boos from the spectators.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Several boxing experts felt Parker, thirty-one, merited the victory but the British boxer suffered the second defeat of his 28-bout professional journey.
The Champion Claims Triumph in Slow Affair
His career development has stalled in recent years through contract problems and extended periods of inactivity.
Once tipped as the next star of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors willing to be aggressive in a tentative initial rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second bout at light-heavyweight.
By the fight's midpoint, Parker had kept it close, making life awkward for Buatsi, who worked behind a solid defense to edge the more effective rounds.
The spectators' acceptance diminished during constant clinching, with Parker going down various instances during grappling.
The underdog ended stronger, but multiple periods were challenging to assess. Nevertheless, previous titleholder the boxing analyst - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just one round.
Buatsi advances, but with the presence of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Hatton Homage Motivates Manchester Derby Fight
In the supporting bout, Arthur overcame Rea by divided decision in a terrific local rivalry battle to win the continental belt.
The arena became quiet for a extremely emotional homage prior to the bout as 10 bells rang in memory of local hero Hatton, who succumbed aged 46 in recently.
It was the first major televised boxing event in the city since Hatton's passing. As the final chime ended, the quiet became a shout of "only one Hatton".
The fighting during the bout equaled the emotion. Arthur was floored by a significant blow squarely on the jaw in the early round, getting up unsteadily to his footing on the eight count.
But the red-colored equipment of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was injured early, heavy swelling developing under his face as the pair exchanged powerful shots.
The challenger, 27, landed power punches but the champion, 34, fought fire with fire with effective uppercuts as the intensity hardly diminished over twelve rounds.
The victor edged the result with judges scorecards of even score, 115-113 and wider gap, but each competitor were relentless and brave, all the qualities their hero the legend represented.
