British light-heavyweight Joshua Buatsi earned a win yet underwhelming beginning to life under fresh promotional backing with a points victory 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 the challenger had his moments, delivering effective clean blows in a fight where the techniques never quite clicked, with frequent clinching marring the action.
The scoring judges had it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my best showing but what defines champions? They prevail," Buatsi commented in the arena as he was met by disapproval from the audience.
The British fighter returned to winning ways after his initial pro loss - 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 contender endured the second loss of his 28-fight career.
The Champion Claims Win in Uneventful Encounter
The fighter's advancement has plateaued in the past few years through contract problems and long spells of lack of fights.
Once tipped as the future of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what feels like a last chance - under recent support.
He began cautiously, with neither fighter prepared to overcommit in a tentative opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had stayed in contention, making life awkward for Buatsi, who operated with a protective stance to narrowly win the better stanzas.
The crowd's patience decreased amid constant clinching, with Parker slipping to the canvas several times during clinches.
Parker ended stronger, but several stanzas were challenging to assess. However, former world champion the boxing analyst - working with Dazn - had it particularly wide for Parker, giving Buatsi just only one stanza.
Buatsi moves on, but with the likes of power puncher Benavidez, and Russian champion Bivol leading the category, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Tribute Motivates All-Manchester Contest
In the secondary fight, Lyndon Arthur defeated Bradley Rea by divided decision in a excellent Manchester vs Manchester contest to win the European light-heavyweight belt.
The stadium fell silent for a deeply poignant memorial before the fight as 10 bells sounded in remembrance of Manchester's own Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised boxing event in the urban center since the passing of Hatton. As the last bell disappeared, the quiet transitioned to a roar of "there's just one Ricky Hatton".
The competition inside the ropes matched the emotion. The eventual winner was sent to the canvas by a left hook directly to the chin in the early round, rising shakily to his position on the referee's count.
But the red-colored equipment of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was injured quickly, noticeable damage appearing under his eye as the pair exchanged powerful shots.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with effective uppercuts as the pace rarely dropped over 12 rounds.
The victor edged the verdict with scoring tallies of draw, 115-113 and 115-112, but the two athletes were relentless and full of heart, all the qualities their idol Hatton represented.
