UK light-heavyweight Joshua Buatsi made a winning yet disappointing beginning to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a competitive 10-round bout at the Co-op Live Arena.
But Parker found success, delivering effective clean blows in a contest where the approaches never quite clicked, with frequent clinching spoiling the contest.
The scoring judges judged it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my best performance but what defines champions? They prevail," Buatsi commented in the ringside as he was faced disapproval from the spectators.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to secure a 20th pro success.
Some ringside observers believed Parker, 31, should have been awarded the decision but the British boxer endured the second loss of his 28-bout career.
Buatsi Earns Victory in Dull Affair
The fighter's advancement has slowed in the past few years through management issues and considerable time of inactivity.
Earlier forecast as the next star of British boxing, he now finds himself trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors prepared to take risks in a cagey initial rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was competing in only his second bout at light heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for the favorite, who utilized a protective stance to edge the better rounds.
The spectators' acceptance wore thin during constant clinching, with the challenger falling to the floor several times during clinches.
Parker finished the stronger, but multiple periods were hard to judge. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor progresses, but with the likes of power puncher Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Homage Motivates Manchester Derby Contest
In the co-main event, Arthur defeated Rea by divided decision in a terrific Manchester vs Manchester battle to win the European light heavyweight championship.
The arena fell silent for a profoundly moving memorial prior to the bout as a bell ceremony tolled in honor of the city's native Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the final chime disappeared, the stillness transitioned to a chant of "there's just one Ricky Hatton".
The competition within the ring reflected the feeling. The eventual winner was sent to the canvas by a significant blow squarely on the chin in the early round, standing uncertainly to his feet on the eight count.
But the traditional red shorts of the fighter soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was marked up quickly, significant bruising forming under his face as the duo landed hard punches.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the action hardly diminished over the full distance.
Arthur edged the verdict with judges scorecards of even score, close margin and wider gap, but both fighters were persistent and brave, everything their idol Hatton represented.
