UK light-heavyweight Buatsi secured a victory yet unimpressive start to his career under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a split decision in a competitive 10-round bout at the Co-op Live Arena.
But Parker found success, connecting with powerful single shots in a contest where the approaches failed to mesh, with excessive holding spoiling the action.
Two judges judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my best showing but what do champions do? They succeed," Buatsi stated in the ringside as he was met by negative reactions from the audience.
The Londoner bounced back after his first professional defeat - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers felt the challenger, thirty-one, should have been awarded the decision but the Derbyshire fighter endured the second defeat of his 28-fight professional journey.
The Champion Secures Win in Uneventful Affair
Buatsi's progress has plateaued in recent times through contract problems and considerable time of inactivity.
Previously predicted as the next star of the British fight scene, he now is positioned trying to realize his potential - maybe in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with the two boxers willing to overcommit in a calculated opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was participating in his second fight at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for Buatsi, who utilized a tight guard to edge the more effective periods.
The crowd's patience diminished during frequent holding, with the challenger falling to the floor several times during clinches.
The underdog finished the stronger, but several stanzas were hard to judge. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Hatton Homage Fuels All-Manchester Contest
In the co-main event, Lyndon Arthur overcame Rea by split-decision in a outstanding Manchester vs Manchester fight to win the European light-heavyweight title.
The stadium became quiet for a profoundly moving memorial ahead of the contest as ten bells sounded in memory of the city's native Ricky Hatton, who died at age 46 in recently.
It was the premier televised fight night in the city since the passing of Hatton. As the final chime disappeared, the silence became a shout of "there's just one Hatton".
The fighting within the ring reflected the emotion. The eventual winner was sent to the canvas by a significant blow squarely on the chin in the second round, getting up unsteadily to his feet on the referee's count.
But the traditional red equipment of Arthur soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was damaged quickly, heavy swelling appearing under his eye as the pair exchanged powerful shots.
Rea, 27, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the action rarely dropped over 12 rounds.
The victor edged the result with official scores of 114-114, 115-113 and clearer margin, but both fighters were determined and brave, exactly what their hero the legend represented.
