UK light-heavyweight Buatsi made a winning yet disappointing start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a competitive 10-round bout at the boxing arena.
But the challenger found success, landing some telling single shots in a contest where the styles didn't complement each other, with excessive holding marring the contest.
The scoring judges judged it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest showing but what defines champions? They win," the victor commented in the arena as he was greeted with negative reactions from the audience.
The British fighter got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts believed the challenger, 31, should have been awarded the decision but the British boxer endured the second loss of his 28-bout professional journey.
The Champion Secures Win in Uneventful Affair
His career development has plateaued in the past few years through contract problems and extended periods of inactivity.
Once tipped as the next star of UK boxing, he now stands trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers prepared to take risks in a cagey opening few rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after damaging his hand during the bout, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, Parker had kept it close, making life awkward for Buatsi, who utilized a tight guard to barely take the better periods.
The crowd's patience diminished during constant clinching, with Parker going down several times during close exchanges.
The underdog closed well, but many rounds were hard to judge. Nevertheless, former world champion the boxing analyst - working with Dazn - had it particularly wide for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - based on this performance - still seems difficult.
Hatton Tribute Fuels Local Rivalry Contest
In the secondary fight, Arthur defeated Bradley Rea by close verdict in a outstanding local rivalry fight to win the continental title.
The venue fell silent for a deeply poignant homage ahead of the contest as 10 bells sounded in memory of local hero Hatton, who died aged 46 in recently.
It was the initial significant broadcast fight night in the urban center since Hatton's death. As the concluding sound faded, the silence gave way to a chant of "only one Hatton".
The action inside the ropes equaled the emotion. Arthur was knocked down by a significant blow flush on the jaw in the second round, rising shakily to his footing on the referee's count.
But the Manchester United-red equipment of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was injured quickly, heavy swelling appearing under his orbital area as the pair landed hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with powerful upward punches as the pace seldom decreased over the full distance.
Arthur narrowly won the decision with judges scorecards of draw, close margin and 115-112, but both fighters were relentless and brave, all the qualities their idol Hatton stood for.
