British light heavyweight Joshua Buatsi secured a victory yet disappointing beginning to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a tight ten-round contest at the Manchester venue.
But Parker found success, landing some telling individual punches in a fight where the approaches never quite clicked, with constant grabbing marring the action.
Two judges had it 96-94 to the winner, with the third a 95-95 draw.
"It wasn't my best performance but what do champions do? They win," the victor stated in the arena as he was met by negative reactions from the crowd.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a 20th pro success.
Various analysts thought Parker, 31, deserved the win but the contender endured the second setback of his 28-match professional journey.
Buatsi Secures Victory in Uneventful Match
Buatsi's progress has slowed in recent times through promotional disputes and long spells of inactivity.
Previously predicted as the future of the British fight scene, he now is positioned trying to realize his potential - maybe in what feels like a last chance - under recent support.
He started carefully, with the two boxers willing to take risks in a tentative early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was making just his second fight at the higher weight class.
By the middle rounds, Parker had made it competitive, creating difficulties for the favorite, who utilized a solid defense to narrowly win the better stanzas.
The crowd's patience diminished amid regular grabbing, with the challenger going down multiple occasions during clinches.
Parker closed well, but many rounds were difficult to score. However, former world champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor progresses, but with the caliber of hard hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Homage Inspires Local Rivalry Contest
In the supporting bout, Arthur overcame Rea by divided decision in a terrific Manchester vs Manchester fight to secure the European light heavyweight championship.
The arena grew hushed for a extremely emotional homage before the fight as 10 bells rang in remembrance of local hero Ricky Hatton, who succumbed 46 years old in recently.
It was the first major televised fight night in the location since Hatton's death. As the concluding sound disappeared, the stillness transitioned to a chant of "there's just one Ricky Hatton".
The competition within the ring matched the sentiment. Arthur was floored by a left hook flush on the face in the second round, standing uncertainly to his feet on the referee's count.
But the traditional red shorts of the fighter soon came forward again. Across the ring, the opponent - in opposing blue - was injured quickly, significant bruising appearing under his orbital area as the pair landed hard punches.
Rea, 27, delivered effective strikes but the champion, thirty-four, responded in kind with powerful upward punches as the pace hardly diminished over 12 rounds.
The victor narrowly won the verdict with judges scorecards of draw, 115-113 and 115-112, but the two athletes were relentless and courageous, all the qualities their hero Hatton represented.
