British light heavyweight Buatsi secured a victory yet underwhelming beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a tight ten-round contest at the boxing arena.
But the challenger landed significant shots, connecting with powerful clean blows in a contest where the styles never quite clicked, with constant grabbing marring the action.
Two judges scored it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my best performance but what do champions do? They win," the victor commented in the ring as he was greeted with disapproval from the crowd.
The Londoner returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to claim a 20th pro success.
Several boxing experts felt the challenger, thirty-one, merited the win but the contender experienced the second setback of his 28-bout professional journey.
Buatsi Earns Victory in Uneventful Match
The fighter's advancement has stalled in recent years through management issues and extended periods of inactivity.
Previously predicted as the rising talent of British boxing, he now finds himself trying to achieve expectations - maybe in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with neither fighter prepared to take risks in a calculated opening few rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was participating in his second bout at light-heavyweight.
By the halfway stage, the underdog had kept it close, creating difficulties for Buatsi, who operated with a solid defense to edge the better stanzas.
The spectators' acceptance diminished amid frequent holding, with Parker slipping to the canvas several times during clinches.
The underdog finished the stronger, but multiple periods were challenging to assess. Although, ex-champion the boxing analyst - working with Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi moves on, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Homage Fuels Local Rivalry Fight
In the secondary fight, Arthur overcame Rea by divided decision in a outstanding local rivalry contest to capture the European light heavyweight championship.
The venue grew hushed for a extremely emotional memorial ahead of the contest as a bell ceremony rang in honor of Manchester's own Ricky Hatton, who died at age 46 in recently.
It was the first major televised boxing event in the location since Hatton's passing. As the last bell faded, the quiet transitioned to a roar of "there's only one Ricky Hatton".
The fighting within the ring matched the emotion. Arthur was knocked down by a powerful punch squarely on the face in the second round, standing uncertainly to his feet on the eight count.
But the Manchester United-red shorts of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was damaged quickly, heavy swelling appearing under his face as the duo exchanged punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, 34, responded in kind with stinging uppercuts as the pace hardly diminished over the full distance.
Arthur edged the verdict with scoring tallies of even score, narrow lead and wider gap, but each competitor were persistent and courageous, everything their idol the legend embodied.
