UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a tight 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a contest where the approaches failed to mesh, with constant grabbing marring the action.
The scoring judges scored it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my greatest performance but what defines champions? They prevail," the victor commented in the ring as he was met by negative reactions from the audience.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in February - to earn a 20th pro success.
Some ringside observers thought the challenger, thirty-one, deserved the decision but the Derbyshire fighter endured the second setback of his 28-bout professional journey.
Buatsi Claims Triumph in Uneventful Encounter
The fighter's advancement has plateaued in recent years through promotional disputes and long spells of lack of fights.
Earlier forecast as the future of the British fight scene, he now is positioned trying to achieve expectations - possibly in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with both competitors willing to take risks in a cagey early stages.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in two years ago after damaging his hand in the contest, was making just his second fight at the higher weight class.
By the fight's midpoint, Parker had made it competitive, creating difficulties for Buatsi, who operated with a protective stance to edge the more effective periods.
The audience's tolerance wore thin amid frequent holding, with Parker going down several times during grappling.
Parker closed well, but multiple periods were hard to judge. However, former world champion Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
Buatsi moves on, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Homage Motivates Local Rivalry Battle
In the supporting bout, Arthur overcame Rea by divided decision in a terrific Manchester vs Manchester contest to secure the European light heavyweight championship.
The stadium fell silent for a deeply poignant tribute prior to the bout as 10 bells sounded in memory of local hero Hatton, who passed away at age 46 in recently.
It was the premier televised fight night in the location since Hatton's death. As the final chime disappeared, the silence gave way to a shout of "there's just one Ricky Hatton".
The fighting during the bout equaled the feeling. Arthur was sent to the canvas by a powerful punch flush on the face in the early round, rising shakily to his feet on the referee's count.
But the Manchester United-red shorts of Arthur soon advanced again. From the other corner, Rea - in rival City blue - was damaged early, significant bruising forming under his eye as the duo traded powerful shots.
The challenger, 27, connected with strong blows but the champion, 34, matched the intensity with stinging uppercuts as the action hardly diminished over twelve rounds.
The victor narrowly won the result with scoring tallies of even score, narrow lead and clearer margin, but each competitor were determined and brave, everything their hero Hatton embodied.
