British light-heavyweight Joshua Buatsi made a winning yet disappointing start to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a tight 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, landing some telling individual punches in a match where the styles didn't complement each other, with constant grabbing marring the contest.
The scoring judges scored it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my finest showing but what do champions do? They win," the victor stated in the arena as he was greeted with boos from the audience.
The Londoner bounced back after his maiden career setback - an thrilling contest with his previous opponent in February - to earn a 20th pro success.
Several boxing experts thought the challenger, thirty-one, merited the win but the contender suffered the second setback of his 28-fight professional journey.
Buatsi Earns Win in Slow Match
His career development has slowed in recent times through contract problems and extended periods of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to realize his potential - possibly in what seems like a final roll of the dice - under recent support.
He began cautiously, with both competitors prepared to overcommit in a cagey initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after breaking his hand mid-fight, was making just his second appearance at the higher weight class.
By the halfway stage, the underdog had kept it close, making life awkward for the favorite, who utilized a solid defense to narrowly win the cleaner periods.
The audience's tolerance decreased during constant clinching, with the challenger falling to the floor multiple occasions during clinches.
Parker finished the stronger, but multiple periods were hard to judge. Although, previous titleholder the boxing analyst - working with Dazn - had it significantly in favor for the challenger, awarding Buatsi just one round.
Buatsi advances, but with the presence of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Memorial Fuels Manchester Derby Contest
In the supporting bout, Arthur overcame Rea by split-decision in a terrific local rivalry fight to capture the continental belt.
The arena became quiet for a profoundly moving memorial prior to the bout as ten bells rang in honor of the city's native Ricky Hatton, who passed away aged 46 in September.
It was the initial significant broadcast fight night in the location since Hatton's death. As the final chime faded, the quiet transitioned to a shout of "there's just one Ricky Hatton".
The competition within the ring matched the sentiment. The eventual winner was floored by a powerful punch directly to the face in the early round, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red attire of the fighter soon advanced again. On the opposite side, the opponent - in opposing blue - was damaged soon, heavy swelling appearing under his orbital area as the duo exchanged punishing blows.
The challenger, 27, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the pace hardly diminished over the full distance.
The victor barely secured the verdict with judges scorecards of 114-114, narrow lead and wider gap, but both fighters were determined and courageous, exactly what their idol the legend stood for.
