UK light heavyweight Buatsi earned a win yet disappointing start to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a tight 10-round contest at the boxing arena.
But the challenger had his moments, delivering effective single shots in a contest where the techniques never quite clicked, with excessive holding disrupting the fight.
The officials scored it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my finest showing but how do champions respond? They win," the victor said in the ring as he was met by boos from the crowd.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to earn a 20th career victory.
Some ringside observers believed Parker, thirty-one, should have been awarded the victory but the British boxer experienced the second setback of his 28-fight career.
The Champion Secures Triumph in Slow Affair
The fighter's advancement has plateaued in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the future of the British fight scene, he now is positioned trying to realize his potential - perhaps in what seems like a last chance - under fresh management.
He began cautiously, with the two boxers ready to take risks in a calculated initial rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was participating in his second fight at the higher weight class.
By the halfway stage, Parker had kept it close, making life awkward for Buatsi, who utilized a solid defense to edge the more effective periods.
The audience's tolerance decreased amid regular grabbing, with the challenger slipping to the canvas various instances during close exchanges.
Parker closed well, but several stanzas were hard to judge. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Hatton Memorial Inspires Manchester Derby Contest
In the secondary fight, Lyndon Arthur beat Bradley Rea by split-decision in a outstanding Manchester vs Manchester fight to secure the European light-heavyweight title.
The venue became quiet for a profoundly moving tribute ahead of the contest as 10 bells tolled in memory of local hero Ricky Hatton, who died aged 46 in September.
It was the premier televised fight night in the location since Hatton's passing. As the last bell disappeared, the silence gave way to a chant of "there's just one Hatton".
The fighting during the bout matched the feeling. Arthur was knocked down by a powerful punch flush on the face in the second round, getting up unsteadily to his position on the referee's count.
But the Manchester United-red attire of Arthur soon pressed ahead again. From the other corner, the opponent - in competitive blue - was marked up early, significant bruising forming under his eye as the fighters landed powerful shots.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, fought fire with fire with effective uppercuts as the intensity rarely dropped over the full distance.
The victor narrowly won the decision with judges scorecards of 114-114, 115-113 and 115-112, but each competitor were persistent and courageous, exactly what their idol the legend stood for.
