British light heavyweight Joshua Buatsi secured a victory yet underwhelming start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a split decision in a tight ten-round bout at the Co-op Live Arena.
But the challenger found success, delivering effective clean blows in a contest where the styles didn't complement each other, with excessive holding marring the fight.
Two judges judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my best showing but how do champions respond? They succeed," Buatsi stated in the ringside as he was faced disapproval from the spectators.
The British fighter bounced back after his maiden career setback - an memorable fight with his previous opponent in February - to earn a twentieth professional win.
Some ringside observers thought the challenger, thirty-one, should have been awarded the victory but the Derbyshire fighter suffered the second setback of his 28-match professional journey.
Buatsi Claims Triumph in Uneventful Affair
Buatsi's progress has plateaued in the past few years through contract problems and long spells of inactivity.
Once tipped as the next star of British boxing, he now is positioned trying to achieve expectations - perhaps in what feels like a final roll of the dice - under fresh management.
He opened tentatively, with both competitors ready to take risks in a tentative initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after damaging his hand in the contest, was participating in his second fight at the higher weight class.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who operated with a solid defense to narrowly win the better stanzas.
The spectators' acceptance diminished amid constant clinching, with the challenger falling to the floor several times during clinches.
Parker ended stronger, but several stanzas were difficult to score. Although, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi moves on, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a outstanding all-Manchester contest to secure the continental title.
The venue grew hushed for a deeply poignant memorial before the fight as a bell ceremony tolled in remembrance of local hero Ricky Hatton, who died at age 46 in last month.
It was the premier televised match card in the city since Hatton's death. As the last bell disappeared, the quiet became a roar of "there's just one Hatton".
The action during the bout reflected the emotion. Arthur was knocked down by a significant blow squarely on the face in the early round, rising shakily to his feet on the eight count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. Across the ring, the opponent - in rival City blue - was damaged soon, significant bruising developing under his orbital area as the fighters exchanged hard punches.
The challenger, 27, landed power punches but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace rarely dropped over the full distance.
Arthur narrowly won the decision with scoring tallies of draw, 115-113 and 115-112, but the two athletes were determined and courageous, all the qualities their inspiration the legend stood for.
