UK light heavyweight Joshua Buatsi made a winning yet unimpressive beginning to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a split decision in a close 10-round fight at the Manchester venue.
But Parker had his moments, connecting with powerful clean blows in a contest where the approaches never quite clicked, with excessive holding spoiling the action.
The officials judged it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my finest performance but what defines champions? They win," the victor commented in the arena as he was greeted with disapproval from the crowd.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts thought Parker, 31, deserved the win but the British boxer experienced the second loss of his 28-bout professional journey.
Buatsi Earns Win in Uneventful Encounter
Buatsi's progress has slowed in recent times through promotional disputes and considerable time of inactivity.
Once tipped as the rising talent of British boxing, he now finds himself trying to realize his potential - maybe in what feels like a ultimate opportunity - under recent support.
He started carefully, with the two boxers prepared to overcommit in a tentative initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in two years ago after suffering a hand injury during the bout, was competing in only his second appearance at light-heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for the favorite, who operated with a protective stance to edge the more effective periods.
The crowd's patience diminished because of regular grabbing, with the challenger slipping to the canvas several times during clinches.
Parker ended stronger, but several stanzas were difficult to score. However, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
Buatsi advances, but with the presence of power puncher David Benavidez, and Russian star Bivol ruling the division, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Tribute Fuels All-Manchester Contest
In the co-main event, Lyndon Arthur overcame Rea by split-decision in a excellent Manchester vs Manchester fight to win the continental belt.
The venue grew hushed for a profoundly moving tribute prior to the bout as ten bells tolled in memory of the city's native Hatton, who succumbed 46 years old in September.
It was the premier televised fight night in the urban center since Hatton's passing. As the final chime faded, the silence gave way to a chant of "there's just one Ricky Hatton".
The competition inside the ropes matched the emotion. Arthur was knocked down by a significant blow squarely on the face in the initial stage, rising shakily to his feet on the referee's count.
But the Manchester United-red equipment of Arthur soon came forward again. Across the ring, the opponent - in opposing blue - was damaged early, significant bruising appearing under his face as the fighters exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with powerful upward punches as the intensity rarely dropped over twelve rounds.
The victor narrowly won the verdict with scoring tallies of even score, 115-113 and wider gap, but both fighters were relentless and courageous, exactly what their inspiration Hatton embodied.
