British light heavyweight Joshua Buatsi earned a win yet unimpressive start to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a split decision in a competitive 10-round bout at the Manchester venue.
But Parker found success, delivering effective individual punches in a contest where the techniques failed to mesh, with frequent clinching spoiling the fight.
The officials judged it 96-94 for to the winner, with the final judge a even score.
"It wasn't my greatest performance but what defines champions? They win," Buatsi commented in the ringside as he was met by disapproval from the spectators.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in February - to earn a 20th pro success.
Some ringside observers thought Parker, 31, should have been awarded the decision but the British boxer endured the second defeat of his 28-bout professional journey.
Buatsi Secures Victory in Uneventful Match
The fighter's advancement has plateaued in recent times through promotional disputes and considerable time of inactivity.
Once tipped as the future of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what seems like a last chance - under recent support.
He began cautiously, with neither fighter willing to overcommit in a cagey initial rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was participating in his second bout at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a protective stance to narrowly win the more effective rounds.
The spectators' acceptance decreased amid constant clinching, with the challenger falling to the floor various instances during clinches.
The underdog ended stronger, but multiple periods were challenging to assess. Nevertheless, ex-champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just one round.
Buatsi moves on, but with the likes of heavy hitter Benavidez, and Russian star Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Homage Fuels All-Manchester Fight
In the secondary fight, Lyndon Arthur defeated Bradley Rea by split-decision in a terrific all-Manchester contest to capture the European light heavyweight championship.
The arena fell silent for a profoundly moving homage ahead of the contest as a bell ceremony tolled in remembrance of local hero Ricky Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the last bell ended, the silence gave way to a roar of "there's just one Hatton".
The fighting within the ring matched the feeling. Arthur was sent to the canvas by a powerful punch flush on the face in the early round, getting up unsteadily to his footing on the referee's count.
But the Manchester United-red shorts of Arthur soon came forward again. From the other corner, the opponent - in competitive blue - was marked up quickly, noticeable damage developing under his orbital area as the duo landed powerful shots.
The challenger, 27, connected with strong blows but Arthur, 34, matched the intensity with stinging uppercuts as the pace hardly diminished over the full distance.
The victor edged the result with judges scorecards of draw, 115-113 and wider gap, but both fighters were determined and brave, all the qualities their idol the legend stood for.
