British light-heavyweight Joshua Buatsi secured a victory yet disappointing beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a split decision in a competitive 10-round bout at the Co-op Live Arena.
But the challenger found success, landing some telling clean blows in a contest where the techniques didn't complement each other, with constant grabbing marring the fight.
The scoring judges scored it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest performance but how do champions respond? They prevail," the victor commented in the ring as he was met by negative reactions from the audience.
The Londoner got back to winning after his initial pro loss - an thrilling contest with his previous opponent in February - to earn a 20th career victory.
Some ringside observers believed Parker, thirty-one, deserved the victory but the contender endured the second defeat of his 28-bout professional journey.
The Champion Claims Triumph in Uneventful Match
His career development has stalled in recent years through promotional disputes and extended periods of lack of fights.
Previously predicted as the future of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors ready to take risks in a calculated opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after damaging his hand during the bout, was participating in his second fight at light heavyweight.
By the halfway stage, Parker had kept it close, causing problems for Buatsi, who worked behind a tight guard to narrowly win the better rounds.
The audience's tolerance wore thin amid constant clinching, with the challenger going down various instances during clinches.
The underdog closed well, but several stanzas were difficult to score. Nevertheless, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the presence of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Hatton Memorial Inspires All-Manchester Contest
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a outstanding all-Manchester fight to win the continental belt.
The venue grew hushed for a deeply poignant memorial ahead of the contest as a bell ceremony tolled in memory of local hero Ricky Hatton, who passed away at age 46 in September.
It was the premier televised boxing event in the urban center since Hatton's death. As the concluding sound ended, the silence became a shout of "there's just one Hatton".
The action during the bout matched the emotion. The eventual winner was sent to the canvas by a powerful punch directly to the face in the early round, standing uncertainly to his feet on the count of eight.
But the Manchester United-red attire of Arthur soon pressed ahead again. From the other corner, Rea - in competitive blue - was injured quickly, heavy swelling developing under his orbital area as the duo exchanged hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with stinging uppercuts as the intensity hardly diminished over twelve rounds.
Arthur narrowly won the result with scoring tallies of draw, narrow lead and 115-112, but the two athletes were determined and full of heart, exactly what their idol Hatton stood for.
