British light heavyweight Joshua Buatsi secured a victory yet disappointing debut to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a competitive 10-round fight at the Co-op Live Arena.
But Parker found success, connecting with powerful individual punches in a fight where the approaches didn't complement each other, with frequent clinching spoiling the fight.
Two judges judged it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my greatest display but what defines champions? They win," the victor said in the ringside as he was faced disapproval from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in February - to earn a 20th career victory.
Various analysts believed the challenger, thirty-one, merited the victory but the British boxer experienced the second setback of his 28-fight career.
Buatsi Claims Triumph in Uneventful Encounter
His career development has stalled in recent times through contract problems and considerable time of lack of fights.
Once tipped as the future of British boxing, he now stands trying to realize his potential - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with both competitors ready to be aggressive in a tentative early stages.
Parker, whose single earlier setback came at super middleweight against John Ryder in two years ago after suffering a hand injury during the bout, was participating in his second fight at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, causing problems for the favorite, who utilized a protective stance to narrowly win the more effective periods.
The spectators' acceptance decreased amid regular grabbing, with the challenger going down several times during close exchanges.
Parker closed well, but several stanzas were challenging to assess. Although, former world champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor advances, but with the caliber of hard hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Hatton Memorial Fuels Local Rivalry Contest
In the co-main event, Lyndon Arthur beat Rea by close verdict in a terrific local rivalry fight to secure the European light heavyweight championship.
The stadium fell silent for a extremely emotional homage prior to the bout as a bell ceremony rang in honor of Manchester's own Hatton, who died aged 46 in September.
It was the initial significant broadcast match card in the city since Hatton's death. As the concluding sound ended, the stillness became a chant of "there's only one Ricky Hatton".
The action inside the ropes matched the sentiment. Arthur was sent to the canvas by a left hook squarely on the face in the early round, getting up unsteadily to his position on the referee's count.
But the Manchester United-red shorts of the fighter soon came forward again. From the other corner, Rea - in competitive blue - was damaged quickly, heavy swelling forming under his eye as the fighters exchanged hard punches.
The challenger, 27, delivered effective strikes but Arthur, 34, responded in kind with effective uppercuts as the pace seldom decreased over the full distance.
The victor narrowly won the decision with official scores of even score, narrow lead and 115-112, but each competitor were determined and full of heart, everything their inspiration Hatton embodied.
