British light-heavyweight Buatsi earned a win yet unimpressive beginning to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a split decision in a tight 10-round bout at the Manchester venue.
But the challenger landed significant shots, delivering effective single shots in a contest where the styles never quite clicked, with constant grabbing marring the fight.
The scoring judges scored it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my finest display but what do champions do? They win," the victor said in the ring as he was met by negative reactions from the spectators.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in February - to secure a 20th pro success.
Various analysts believed Parker, 31, deserved the victory but the British boxer endured the second loss of his 28-match professional journey.
The Champion Earns Triumph in Slow Affair
The fighter's advancement has stalled in recent years through contract problems and long spells of inactivity.
Previously predicted as the next star of British boxing, he now is positioned trying to realize his potential - perhaps in what feels like a last chance - under new promotional backing.
He began cautiously, with the two boxers ready to overcommit in a calculated opening few rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second bout at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, making life awkward for Buatsi, who operated with a protective stance to narrowly win the more effective periods.
The crowd's patience diminished during constant clinching, with Parker falling to the floor multiple occasions during clinches.
Parker ended stronger, but multiple periods were hard to judge. Nevertheless, ex-champion the boxing analyst - working with Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the presence of heavy hitter Benavidez, and Russian champion Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Homage Inspires All-Manchester Contest
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a outstanding Manchester vs Manchester battle to capture the European light-heavyweight championship.
The arena grew hushed for a profoundly moving memorial ahead of the contest as ten bells rang in memory of Manchester's own Ricky Hatton, who passed away 46 years old in last month.
It was the premier televised fight night in the location since Hatton's death. As the last bell ended, the silence became a roar of "there's only one Ricky Hatton".
The competition during the bout reflected the sentiment. Arthur was floored by a significant blow directly to the face in the initial stage, standing uncertainly to his feet on the eight count.
But the red-colored attire of the fighter soon pressed ahead again. On the opposite side, Rea - in opposing blue - was marked up soon, heavy swelling forming under his eye as the fighters landed hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the pace seldom decreased over 12 rounds.
Arthur edged the result with official scores of draw, 115-113 and wider gap, but each competitor were persistent and full of heart, all the qualities their inspiration Hatton embodied.
