UK light-heavyweight Joshua Buatsi earned a win yet unimpressive beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a close ten-round fight at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling single shots in a contest where the approaches failed to mesh, with excessive holding marring the action.
The officials scored it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest showing but what defines champions? They succeed," the victor commented in the ring as he was greeted with negative reactions from the spectators.
The Londoner returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in February - to secure a 20th career victory.
Several boxing experts believed the challenger, thirty-one, merited the decision but the British boxer endured the second loss of his 28-fight career.
Buatsi Claims Victory in Uneventful Encounter
His career development has plateaued in recent years through management issues and considerable time of inactivity.
Previously predicted as the next star of British boxing, he now finds himself trying to realize his potential - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors prepared to overcommit in a tentative early stages.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after breaking his hand during the bout, was competing in only his second fight at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to barely take the cleaner periods.
The crowd's patience diminished during regular grabbing, with Parker slipping to the canvas various instances during clinches.
Parker ended stronger, but several stanzas were difficult to score. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, giving Buatsi just only one stanza.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Tribute Motivates Local Rivalry Battle
In the secondary fight, Arthur beat Rea by divided decision in a outstanding local rivalry battle to win the continental belt.
The stadium grew hushed for a deeply poignant tribute prior to the bout as ten bells sounded in memory of local hero Hatton, who passed away 46 years old in September.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the last bell ended, the stillness became a shout of "only one Hatton".
The action during the bout matched the emotion. The eventual winner was sent to the canvas by a powerful punch directly to the face in the initial stage, rising shakily to his feet on the referee's count.
But the red-colored equipment of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was damaged quickly, significant bruising forming under his face as the pair traded hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity seldom decreased over the full distance.
The victor edged the result with official scores of 114-114, 115-113 and wider gap, but both fighters were relentless and brave, everything their idol Hatton represented.
