UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a close 10-round fight at the Co-op Live Arena.
But Parker found success, connecting with powerful individual punches in a match where the approaches didn't complement each other, with frequent clinching spoiling the fight.
The scoring judges scored it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my best display but how do champions respond? They prevail," the victor stated in the ringside as he was met by boos from the spectators.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in last winter - to earn a 20th pro success.
Some ringside observers thought Parker, 31, merited the decision but the contender endured the second setback of his 28-bout career.
The Champion Claims Triumph in Uneventful Affair
His career development has plateaued in recent times through promotional disputes and long spells of lack of fights.
Once tipped as the future of UK boxing, he now finds himself trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with neither fighter prepared to take risks in a calculated early stages.
The challenger, whose sole prior loss came at super middleweight against John Ryder in two years ago after suffering a hand injury during the bout, was making just his second appearance at the higher weight class.
By the middle rounds, Parker had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to barely take the more effective periods.
The spectators' acceptance diminished amid constant clinching, with Parker going down several times during grappling.
The underdog closed well, but several stanzas were hard to judge. Although, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just a single round.
Buatsi moves on, but with the presence of power puncher David Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - judging by this showing - still appears challenging.
Hatton Memorial Inspires Manchester Derby Fight
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a outstanding local rivalry fight to secure the continental championship.
The arena became quiet for a profoundly moving tribute prior to the bout as ten bells tolled in memory of Manchester's own Hatton, who passed away at age 46 in last month.
It was the premier televised boxing event in the city since the passing of Hatton. As the concluding sound disappeared, the quiet transitioned to a chant of "there's just one Ricky Hatton".
The fighting inside the ropes reflected the sentiment. The eventual winner was sent to the canvas by a powerful punch flush on the chin in the second round, rising shakily to his footing on the count of eight.
But the Manchester United-red equipment of Arthur soon advanced again. On the opposite side, the opponent - in rival City blue - was damaged soon, heavy swelling forming under his face as the fighters landed hard punches.
The challenger, 27, connected with strong blows but the champion, 34, fought fire with fire with effective uppercuts as the intensity hardly diminished over twelve rounds.
Arthur barely secured the verdict with judges scorecards of even score, narrow lead and wider gap, but the two athletes were relentless and full of heart, everything their inspiration the legend stood for.
