British light-heavyweight Joshua Buatsi secured a victory yet unimpressive debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a tight 10-round bout at the Co-op Live Arena.
But Parker found success, delivering effective individual punches in a contest where the styles never quite clicked, with constant grabbing disrupting the fight.
The officials scored it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my greatest performance but what defines champions? They prevail," Buatsi commented in the ringside as he was met by boos from the spectators.
The British fighter bounced back after his first professional defeat - an memorable fight with Callum Smith in February - to earn a 20th pro success.
Some ringside observers thought Parker, 31, merited the win but the Derbyshire fighter experienced the second loss of his 28-fight career.
The Champion Secures Triumph in Slow Affair
The fighter's advancement has plateaued in the past few years through management issues and considerable time of lack of fights.
Once tipped as the future of the British fight scene, he now stands trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter willing to overcommit in a tentative initial rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in two years ago after damaging his hand in the contest, was competing in only his second bout at light heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who worked behind a solid defense to narrowly win the cleaner rounds.
The audience's tolerance decreased because of constant clinching, with Parker slipping to the canvas several times during close exchanges.
The underdog ended stronger, but multiple periods were challenging to assess. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor progresses, but with the presence of hard hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Hatton Homage Fuels Manchester Derby Fight
In the supporting bout, Arthur overcame Rea by close verdict in a terrific local rivalry fight to secure the European light heavyweight championship.
The venue grew hushed for a deeply poignant tribute prior to the bout as ten bells rang in remembrance of local hero Hatton, who passed away at age 46 in September.
It was the first major televised match card in the urban center since Hatton's death. As the concluding sound ended, the silence gave way to a shout of "there's just one Ricky Hatton".
The competition during the bout equaled the emotion. The eventual winner was knocked down by a left hook squarely on the chin in the early round, rising shakily to his feet on the referee's count.
But the traditional red attire of Arthur soon pressed ahead again. From the other corner, Rea - in opposing blue - was damaged quickly, heavy swelling appearing under his orbital area as the fighters exchanged punishing blows.
The challenger, 27, landed power punches but Arthur, 34, matched the intensity with effective uppercuts as the pace hardly diminished over 12 rounds.
Arthur edged the result with official scores of even score, narrow lead and clearer margin, but both fighters were determined and full of heart, exactly what their idol the legend embodied.
