British light-heavyweight Joshua Buatsi earned a win yet unimpressive beginning to life under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a competitive ten-round fight at the Manchester venue.
But Parker found success, delivering effective clean blows in a fight where the approaches didn't complement each other, with constant grabbing disrupting the fight.
The scoring judges judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest performance but what defines champions? They succeed," the victor commented in the arena as he was met by negative reactions from the crowd.
The British fighter bounced back after his maiden career setback - an thrilling contest with his previous opponent in February - to secure a twentieth professional win.
Various analysts thought the challenger, 31, merited the victory but the British boxer experienced the second defeat of his 28-fight career.
The Champion Claims Triumph in Uneventful Affair
His career development has plateaued in the past few years through contract problems and considerable time of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to realize his potential - maybe in what feels like a final roll of the dice - under fresh management.
He began cautiously, with the two boxers willing to take risks in a tentative initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second bout at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for the favorite, who utilized a protective stance to edge the more effective stanzas.
The spectators' acceptance decreased during frequent holding, with the challenger falling to the floor several times during close exchanges.
Parker ended stronger, but multiple periods were hard to judge. Although, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
The victor advances, but with the caliber of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Tribute Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a outstanding local rivalry battle to capture the continental belt.
The venue fell silent for a extremely emotional homage ahead of the contest as 10 bells rang in honor of Manchester's own Hatton, who died aged 46 in September.
It was the first major televised match card in the location since Hatton's death. As the final chime disappeared, the stillness gave way to a roar of "there's only one Hatton".
The action during the bout matched the feeling. Arthur was floored by a powerful punch directly to the chin in the initial stage, standing uncertainly to his footing on the count of eight.
But the Manchester United-red attire of the fighter soon came forward again. From the other corner, the opponent - in rival City blue - was injured soon, significant bruising appearing under his eye as the pair exchanged powerful shots.
The challenger, twenty-seven, landed power punches but Arthur, 34, responded in kind with stinging uppercuts as the intensity rarely dropped over 12 rounds.
Arthur barely secured the decision with scoring tallies of draw, 115-113 and clearer margin, but both fighters were determined and full of heart, all the qualities their hero the legend represented.
