British light heavyweight Buatsi earned a win yet underwhelming beginning to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a close 10-round bout at the boxing arena.
But the challenger had his moments, connecting with powerful individual punches in a fight where the techniques failed to mesh, with frequent clinching spoiling the action.
The scoring judges scored it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my finest display but what defines champions? They win," the victor stated in the ring as he was met by boos from the spectators.
The Londoner returned to winning ways after his initial pro loss - an epic battle with Callum Smith in February - to claim a 20th career victory.
Some ringside observers felt the challenger, 31, merited the victory but the contender endured the second setback of his 28-bout professional journey.
Buatsi Earns Triumph in Slow Match
Buatsi's progress has stalled in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the next star of UK boxing, he now is positioned trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under fresh management.
He opened tentatively, with the two boxers ready to be aggressive in a tentative initial rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after damaging his hand mid-fight, was making just his second appearance at the higher weight class.
By the fight's midpoint, Parker had kept it close, causing problems for the favorite, who worked behind a tight guard to barely take the cleaner stanzas.
The crowd's patience diminished during constant clinching, with Parker going down several times during clinches.
Parker closed well, but multiple periods were difficult to score. Although, previous titleholder Carl Frampton - working with Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
Buatsi progresses, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Hatton Tribute Motivates Manchester Derby Contest
In the secondary fight, Arthur defeated Rea by close verdict in a terrific Manchester vs Manchester fight to capture the European light-heavyweight title.
The arena became quiet for a profoundly moving homage before the fight as 10 bells sounded in honor of the city's native Hatton, who succumbed at age 46 in September.
It was the premier televised boxing event in the urban center since Hatton's passing. As the last bell faded, the silence gave way to a roar of "there's only one Hatton".
The fighting within the ring reflected the feeling. Arthur was sent to the canvas by a powerful punch flush on the chin in the early round, standing uncertainly to his feet on the referee's count.
But the red-colored attire of the fighter soon came forward again. Across the ring, Rea - in opposing blue - was injured soon, significant bruising forming under his face as the pair exchanged powerful shots.
The challenger, 27, landed power punches but the champion, thirty-four, matched the intensity with stinging uppercuts as the action hardly diminished over 12 rounds.
The victor edged the result with official scores of draw, close margin and clearer margin, but the two athletes were determined and courageous, everything their idol the legend stood for.
