UK light heavyweight Joshua Buatsi made a winning yet disappointing debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a close 10-round fight at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful clean blows in a fight where the approaches failed to mesh, with excessive holding disrupting the contest.
The officials judged it 96-94 to Buatsi, with the third a even score.
"It wasn't my best showing but what defines champions? They succeed," Buatsi said in the arena as he was greeted with negative reactions from the audience.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Several boxing experts believed Parker, thirty-one, deserved the decision but the Derbyshire fighter suffered the second defeat of his 28-fight professional journey.
The Champion Earns Victory in Uneventful Encounter
His career development has plateaued in recent years through promotional disputes and long spells of inactivity.
Once tipped as the rising talent of UK boxing, he now finds himself trying to realize his potential - maybe in what feels like a last chance - under new promotional backing.
He started carefully, with the two boxers prepared to take risks in a tentative opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in two years ago after breaking his hand during the bout, was participating in his second appearance at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for Buatsi, who utilized a tight guard to edge the more effective periods.
The spectators' acceptance diminished amid regular grabbing, with Parker falling to the floor several times during grappling.
Parker ended stronger, but several stanzas were difficult to score. Nevertheless, ex-champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just one round.
Buatsi progresses, but with the caliber of hard hitter Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Homage Fuels Manchester Derby Contest
In the secondary fight, Arthur overcame Rea by close verdict in a terrific all-Manchester fight to win the European light-heavyweight championship.
The venue grew hushed for a extremely emotional homage ahead of the contest as a bell ceremony rang in memory of Manchester's own Hatton, who succumbed at age 46 in recently.
It was the premier televised fight night in the urban center since the passing of Hatton. As the final chime faded, the silence gave way to a roar of "only one Hatton".
The fighting during the bout reflected the feeling. The eventual winner was floored by a left hook directly to the jaw in the early round, standing uncertainly to his position on the referee's count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. Across the ring, Rea - in competitive blue - was injured soon, significant bruising appearing under his face as the duo exchanged hard punches.
The challenger, 27, connected with strong blows but the champion, thirty-four, responded in kind with powerful upward punches as the intensity rarely dropped over the full distance.
Arthur narrowly won the result with judges scorecards of 114-114, narrow lead and 115-112, but both fighters were relentless and full of heart, exactly what their idol Hatton embodied.
