UK light-heavyweight Buatsi made a winning yet disappointing debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a tight ten-round contest at the boxing arena.
But Parker had his moments, connecting with powerful single shots in a contest where the styles never quite clicked, with frequent clinching spoiling the contest.
The scoring judges judged it 96-94 to Buatsi, with the third a even score.
"It wasn't my finest showing but what defines champions? They win," the victor said in the ringside as he was greeted with boos from the crowd.
The British fighter got back to winning after his maiden career setback - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Some ringside observers felt Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter experienced the second setback of his 28-fight career.
The Champion Claims Win in Dull Encounter
His career development has plateaued in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to realize his potential - perhaps in what seems like a ultimate opportunity - under fresh management.
He started carefully, with the two boxers willing to take risks in a tentative opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in two years ago after damaging his hand during the bout, was making just his second appearance at light heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for the favorite, who worked behind a tight guard to edge the cleaner stanzas.
The spectators' acceptance decreased because of frequent holding, with Parker falling to the floor multiple occasions during close exchanges.
The underdog closed well, but several stanzas were difficult to score. Nevertheless, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for the challenger, awarding Buatsi just one round.
Buatsi moves on, but with the likes of heavy hitter David Benavidez, and Russian star Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Homage Inspires All-Manchester Fight
In the co-main event, Arthur defeated Rea by divided decision in a excellent Manchester vs Manchester battle to secure the European light heavyweight belt.
The venue became quiet for a profoundly moving memorial before the fight as a bell ceremony tolled in remembrance of local hero Ricky Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast match card in the location since Hatton's death. As the concluding sound faded, the stillness became a shout of "there's only one Ricky Hatton".
The fighting during the bout equaled the sentiment. Arthur was sent to the canvas by a left hook directly to the chin in the second round, standing uncertainly to his feet on the referee's count.
But the red-colored attire of the fighter soon advanced again. On the opposite side, Rea - in opposing blue - was injured quickly, significant bruising forming under his eye as the duo landed powerful shots.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with effective uppercuts as the intensity seldom decreased over twelve rounds.
Arthur narrowly won the result with judges scorecards of even score, close margin and 115-112, but both fighters were determined and full of heart, exactly what their idol Hatton stood for.
