UK light-heavyweight Joshua Buatsi made a winning yet unimpressive start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a competitive 10-round bout at the Manchester venue.
But the challenger landed significant shots, connecting with powerful clean blows in a contest where the techniques didn't complement each other, with constant grabbing spoiling the action.
The scoring judges had it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They prevail," the victor stated in the ring as he was greeted with negative reactions from the spectators.
The British fighter got back to winning after his first professional defeat - an memorable fight with his previous opponent in last winter - to earn a 20th career victory.
Various analysts felt Parker, thirty-one, deserved the victory but the Derbyshire fighter endured the second loss of his 28-fight career.
The Champion Claims Triumph in Slow Encounter
Buatsi's progress has plateaued in the past few years through contract problems and long spells of lack of fights.
Previously predicted as the rising talent of British boxing, he now finds himself trying to realize his potential - maybe in what seems like a last chance - under recent support.
He started carefully, with both competitors ready to be aggressive in a cagey early stages.
Parker, whose sole prior loss came at the lower weight class against his former opponent in two years ago after suffering a hand injury during the bout, was making just his second bout at light-heavyweight.
By the middle rounds, the underdog had kept it close, creating difficulties for the favorite, who utilized a protective stance to edge the more effective stanzas.
The spectators' acceptance diminished during regular grabbing, with Parker slipping to the canvas multiple occasions during clinches.
Parker finished the stronger, but multiple periods were difficult to score. However, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, awarding Buatsi just one round.
Buatsi advances, but with the presence of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - based on this performance - still looks steep.
Hatton Tribute Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur overcame Rea by divided decision in a excellent all-Manchester battle to secure the European light-heavyweight belt.
The arena grew hushed for a profoundly moving tribute prior to the bout as 10 bells sounded in remembrance of Manchester's own Hatton, who died aged 46 in last month.
It was the premier televised fight night in the urban center since the passing of Hatton. As the concluding sound disappeared, the stillness gave way to a roar of "only one Ricky Hatton".
The action inside the ropes reflected the emotion. The eventual winner was sent to the canvas by a significant blow directly to the face in the initial stage, getting up unsteadily to his feet on the count of eight.
But the traditional red shorts of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was marked up quickly, significant bruising developing under his orbital area as the fighters landed punishing blows.
Rea, 27, landed power punches but Arthur, thirty-four, matched the intensity with powerful upward punches as the intensity seldom decreased over twelve rounds.
The victor barely secured the result with scoring tallies of draw, close margin and 115-112, but the two athletes were persistent and brave, everything their hero the legend stood for.
