UK light-heavyweight Buatsi earned a win yet underwhelming debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a close 10-round contest at the Manchester venue.
But the challenger found success, connecting with powerful clean blows in a match where the techniques failed to mesh, with excessive holding marring the action.
The officials judged it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my finest performance but how do champions respond? They prevail," Buatsi said in the ringside as he was met by boos from the crowd.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in last winter - to secure a 20th pro success.
Various analysts believed the challenger, thirty-one, deserved the decision but the British boxer suffered the second loss of his 28-fight career.
Buatsi Secures Victory in Dull Affair
The fighter's advancement has slowed in the past few years through contract problems and considerable time of inactivity.
Previously predicted as the rising talent of British boxing, he now stands trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under recent support.
He started carefully, with neither fighter ready to take risks in a calculated early stages.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was making just his second fight at the higher weight class.
By the halfway stage, Parker had made it competitive, making life awkward for Buatsi, who utilized a solid defense to edge the more effective stanzas.
The spectators' acceptance wore thin during constant clinching, with the challenger falling to the floor various instances during clinches.
The underdog closed well, but several stanzas were difficult to score. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Hatton Tribute Fuels Manchester Derby Contest
In the secondary fight, Arthur overcame Rea by divided decision in a terrific Manchester vs Manchester fight to secure the European light-heavyweight championship.
The arena became quiet for a deeply poignant memorial prior to the bout as 10 bells rang in remembrance of local hero Hatton, who passed away at age 46 in recently.
It was the premier televised fight night in the city since Hatton's passing. As the final chime ended, the silence transitioned to a roar of "only one Hatton".
The fighting within the ring equaled the feeling. The eventual winner was floored by a left hook flush on the chin in the initial stage, standing uncertainly to his feet on the referee's count.
But the red-colored equipment of the fighter soon pressed ahead again. From the other corner, the opponent - in rival City blue - was injured early, noticeable damage developing under his face as the duo exchanged hard punches.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the action rarely dropped over 12 rounds.
The victor narrowly won the verdict with judges scorecards of even score, narrow lead and 115-112, but each competitor were relentless and brave, everything their inspiration Hatton stood for.
