UK light heavyweight Joshua Buatsi made a winning yet disappointing start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a split decision in a close ten-round fight at the Manchester venue.
But the challenger found success, delivering effective clean blows in a fight where the techniques failed to mesh, with excessive holding spoiling the fight.
Two judges judged it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my greatest performance but what do champions do? They win," the victor said in the ringside as he was met by boos from the spectators.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a 20th pro success.
Various analysts felt the challenger, thirty-one, deserved the victory but the British boxer endured the second loss of his 28-fight career.
The Champion Secures Win in Uneventful Match
The fighter's advancement has stalled in recent years through promotional disputes and long spells of inactivity.
Once tipped as the future of British boxing, he now stands trying to achieve expectations - possibly in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with neither fighter willing to take risks in a calculated opening few rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in two years ago after damaging his hand in the contest, was making just his second appearance at the higher weight class.
By the halfway stage, the underdog had kept it close, making life awkward for Buatsi, who utilized a protective stance to edge the cleaner rounds.
The spectators' acceptance diminished amid regular grabbing, with the challenger slipping to the canvas several times during clinches.
The underdog finished the stronger, but multiple periods were hard to judge. Although, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor advances, but with the caliber of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still seems difficult.
Hatton Homage Motivates All-Manchester Fight
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a outstanding Manchester vs Manchester battle to win the continental title.
The arena became quiet for a profoundly moving memorial before the fight as ten bells tolled in remembrance of Manchester's own Ricky Hatton, who succumbed aged 46 in recently.
It was the first major televised match card in the urban center since Hatton's passing. As the last bell ended, the quiet gave way to a shout of "only one Ricky Hatton".
The action inside the ropes reflected the sentiment. The eventual winner was floored by a significant blow squarely on the face in the early round, getting up unsteadily to his footing on the eight count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. Across the ring, Rea - in competitive blue - was damaged quickly, significant bruising appearing under his face as the duo traded punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the pace hardly diminished over twelve rounds.
Arthur barely secured the decision with scoring tallies of 114-114, 115-113 and 115-112, but the two athletes were determined and full of heart, everything their hero Hatton stood for.
