UK light-heavyweight Joshua Buatsi made a winning yet unimpressive debut to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a split decision in a competitive ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful single shots in a fight where the techniques never quite clicked, with excessive holding disrupting the action.
The officials scored it 96-94 to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but what do champions do? They prevail," the victor commented in the arena as he was met by negative reactions from the spectators.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to secure a 20th pro success.
Several boxing experts thought Parker, 31, deserved the decision but the contender suffered the second loss of his 28-fight professional journey.
Buatsi Secures Victory in Slow Match
Buatsi's progress has plateaued in recent times through promotional disputes and considerable time of inactivity.
Once tipped as the future of the British fight scene, he now stands trying to realize his potential - possibly in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with the two boxers prepared to overcommit in a cagey initial rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for the favorite, who worked behind a tight guard to narrowly win the better rounds.
The audience's tolerance decreased amid frequent holding, with Parker falling to the floor several times during close exchanges.
The underdog closed well, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor advances, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Memorial Motivates Manchester Derby Contest
In the co-main event, Arthur overcame Rea by split-decision in a outstanding all-Manchester fight to secure the European light heavyweight title.
The venue grew hushed for a profoundly moving homage before the fight as a bell ceremony tolled in remembrance of the city's native Hatton, who succumbed at age 46 in last month.
It was the premier televised fight night in the city since Hatton's passing. As the concluding sound ended, the stillness became a roar of "there's only one Hatton".
The action inside the ropes equaled the feeling. Arthur was sent to the canvas by a left hook flush on the face in the early round, standing uncertainly to his footing on the eight count.
But the traditional red shorts of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was damaged soon, noticeable damage appearing under his face as the duo traded hard punches.
Rea, 27, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the pace hardly diminished over twelve rounds.
The victor barely secured the result with official scores of 114-114, 115-113 and 115-112, but both fighters were determined and brave, exactly what their idol the legend represented.
