UK light-heavyweight Joshua Buatsi secured a victory yet disappointing beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a close ten-round contest at the Co-op Live Arena.
But the challenger found success, landing some telling individual punches in a match where the styles failed to mesh, with constant grabbing marring the fight.
The officials judged it 96-94 for to Buatsi, with the third a even score.
"It wasn't my greatest performance but how do champions respond? They prevail," Buatsi stated in the ringside as he was met by disapproval from the spectators.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a 20th career victory.
Several boxing experts felt the challenger, 31, deserved the win but the British boxer suffered the second loss of his 28-fight professional journey.
The Champion Secures Victory in Uneventful Affair
The fighter's advancement has plateaued in recent times through management issues and extended periods of lack of fights.
Once tipped as the next star of UK boxing, he now stands trying to realize his potential - possibly in what appears to be a last chance - under new promotional backing.
He opened tentatively, with both competitors ready to be aggressive in a calculated opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second fight at light-heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for Buatsi, who worked behind a tight guard to barely take the cleaner rounds.
The crowd's patience decreased because of regular grabbing, with the challenger going down several times during close exchanges.
Parker closed well, but multiple periods were difficult to score. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just one round.
The victor moves on, but with the caliber of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Memorial Inspires Manchester Derby Battle
In the supporting bout, Arthur overcame Rea by divided decision in a outstanding all-Manchester contest to win the European light heavyweight championship.
The venue became quiet for a profoundly moving memorial prior to the bout as a bell ceremony tolled in memory of Manchester's own Hatton, who passed away aged 46 in last month.
It was the first major televised boxing event in the location since Hatton's death. As the concluding sound faded, the stillness transitioned to a roar of "there's only one Hatton".
The action inside the ropes matched the feeling. Arthur was floored by a left hook directly to the chin in the second round, rising shakily to his position on the referee's count.
But the red-colored attire of Arthur soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was injured quickly, noticeable damage appearing under his eye as the fighters traded powerful shots.
Rea, 27, connected with strong blows but Arthur, 34, fought fire with fire with powerful upward punches as the intensity hardly diminished over 12 rounds.
The victor narrowly won the result with judges scorecards of 114-114, 115-113 and wider gap, but each competitor were persistent and full of heart, exactly what their idol the legend stood for.
