UK light heavyweight Joshua Buatsi earned a win yet underwhelming beginning to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a close 10-round bout at the Co-op Live Arena.
But Parker found success, connecting with powerful single shots in a contest where the styles never quite clicked, with frequent clinching spoiling the action.
The officials had it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my best display but what do champions do? They succeed," the victor said in the ringside as he was faced boos from the audience.
The British fighter got back to winning after his initial pro loss - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Various analysts thought the challenger, 31, should have been awarded the decision but the British boxer endured the second loss of his 28-match career.
Buatsi Earns Triumph in Slow Match
His career development has plateaued in the past few years 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 - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter ready to overcommit in a cagey opening few rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, creating difficulties for the favorite, who utilized a protective stance to narrowly win the better rounds.
The crowd's patience diminished because of constant clinching, with the challenger slipping to the canvas several times during grappling.
Parker finished the stronger, but many rounds were challenging to assess. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it significantly in favor for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Russian star Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Homage Fuels Manchester Derby Battle
In the secondary fight, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific Manchester vs Manchester battle to capture the European light heavyweight championship.
The stadium became quiet for a deeply poignant memorial prior to the bout as 10 bells tolled in honor of the city's native Hatton, who died aged 46 in recently.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the concluding sound disappeared, the quiet became a shout of "there's just one Ricky Hatton".
The action during the bout equaled the emotion. Arthur was floored by a significant blow squarely on the face in the initial stage, getting up unsteadily to his footing on the referee's count.
But the traditional red equipment of the fighter soon came forward again. From the other corner, Rea - in competitive blue - was damaged early, significant bruising developing under his orbital area as the duo landed punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the intensity hardly diminished over twelve rounds.
Arthur barely secured the decision with judges scorecards of even score, close margin and 115-112, but both fighters were determined and courageous, exactly what their inspiration the legend stood for.
