UK light-heavyweight Joshua Buatsi earned a win yet underwhelming start to his career under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a tight 10-round bout at the Manchester venue.
But the challenger had his moments, connecting with powerful single shots in a contest where the techniques never quite clicked, with frequent clinching marring the action.
The officials judged it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my best showing but what do champions do? They succeed," the victor said in the arena as he was met by boos from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with his previous opponent in February - to earn a 20th pro success.
Some ringside observers believed Parker, 31, should have been awarded the win but the contender endured the second defeat of his 28-fight career.
The Champion Secures Win in Slow Encounter
His career development has stalled in recent years through promotional disputes and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now is positioned trying to realize his potential - possibly in what appears to be a last chance - under new promotional backing.
He began cautiously, with both competitors willing to overcommit in a cagey initial rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in two years ago after breaking his hand mid-fight, was making just his second fight at light heavyweight.
By the halfway stage, the underdog had kept it close, making life awkward for Buatsi, who worked behind a tight guard to narrowly win the cleaner rounds.
The spectators' acceptance decreased because of regular grabbing, with Parker slipping to the canvas several times during clinches.
Parker closed well, but several stanzas were challenging to assess. Although, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
The victor progresses, but with the likes of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a excellent local rivalry contest to secure the continental championship.
The venue grew hushed for a profoundly moving memorial ahead of the contest as a bell ceremony rang in remembrance of the city's native Ricky Hatton, who died 46 years old in September.
It was the first major televised fight night in the city since Hatton's death. As the concluding sound ended, the stillness gave way to a roar of "there's only one Ricky Hatton".
The fighting during the bout equaled the emotion. The eventual winner was sent to the canvas by a left hook squarely on the chin in the early round, standing uncertainly to his feet on the referee's count.
But the Manchester United-red equipment of the fighter soon advanced again. Across the ring, the opponent - in rival City blue - was marked up quickly, significant bruising forming under his face as the duo exchanged punishing blows.
The challenger, 27, connected with strong blows but the champion, thirty-four, responded in kind with effective uppercuts as the action hardly diminished over 12 rounds.
Arthur edged the decision with scoring tallies of 114-114, close margin and clearer margin, but each competitor were determined and brave, exactly what their idol Hatton stood for.
