UK light heavyweight Joshua Buatsi secured a victory yet underwhelming start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a tight 10-round bout at the boxing arena.
But Parker had his moments, connecting with powerful single shots in a fight where the styles never quite clicked, with constant grabbing spoiling the contest.
The officials scored it 96-94 to the winner, with the third a 95-95.
"It wasn't my best showing but what defines champions? They prevail," Buatsi said in the ring as he was faced negative reactions from the spectators.
The Londoner bounced back after his maiden career setback - an memorable fight with Callum Smith in February - to claim a 20th career victory.
Several boxing experts thought Parker, 31, merited the win but the contender suffered the second setback of his 28-bout career.
The Champion Secures Triumph in Slow Affair
The fighter's advancement has slowed in recent times through contract problems and considerable time of inactivity.
Once tipped as the future of UK boxing, he now is positioned trying to fulfil that early promise - possibly in what appears to be a last chance - under recent support.
He began cautiously, with the two boxers ready to take risks in a tentative early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after damaging his hand mid-fight, was participating in his second fight at light-heavyweight.
By the halfway stage, Parker had made it competitive, making life awkward for the favorite, who worked behind a tight guard to barely take the more effective stanzas.
The audience's tolerance decreased amid constant clinching, with the challenger going down various instances during clinches.
The underdog finished the stronger, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
Buatsi progresses, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still appears challenging.
Hatton Memorial Inspires Manchester Derby Fight
In the co-main event, Arthur overcame Rea by divided decision in a terrific all-Manchester battle to secure the European light heavyweight title.
The venue grew hushed for a deeply poignant tribute prior to the bout as ten bells sounded in honor of the city's native Ricky Hatton, who died aged 46 in last month.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the last bell disappeared, the stillness transitioned to a chant of "only one Ricky Hatton".
The fighting during the bout equaled the emotion. Arthur was sent to the canvas by a left hook flush on the chin in the early round, getting up unsteadily to his position on the referee's count.
But the red-colored shorts of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was damaged soon, noticeable damage forming under his eye as the duo landed punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with powerful upward punches as the action rarely dropped over the full distance.
The victor narrowly won the result with judges scorecards of draw, 115-113 and wider gap, but both fighters were relentless and courageous, exactly what their idol Hatton stood for.
