UK light heavyweight Buatsi made a winning yet disappointing beginning to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a judges' verdict in a competitive ten-round bout at the Co-op Live Arena.
But Parker landed significant shots, landing some telling single shots in a contest where the approaches never quite clicked, with frequent clinching disrupting the action.
The scoring judges scored it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They succeed," the victor stated in the arena as he was met by negative reactions from the audience.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Several boxing experts thought Parker, thirty-one, deserved the decision but the Derbyshire fighter experienced the second setback of his 28-match career.
The Champion Earns Triumph in Slow Match
Buatsi's progress has plateaued in recent years through promotional disputes and long spells of lack of fights.
Earlier forecast as the next star of the British fight scene, he now stands 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 ready to overcommit in a cagey early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after breaking his hand during the bout, was participating in his second appearance at the higher weight class.
By the halfway stage, Parker had stayed in contention, creating difficulties for Buatsi, who worked behind a tight guard to barely take the cleaner stanzas.
The audience's tolerance diminished amid frequent holding, with Parker falling to the floor multiple occasions during grappling.
Parker ended stronger, but many rounds were challenging to assess. However, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
The victor moves on, but with the presence of hard hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Hatton Memorial Motivates Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a excellent local rivalry contest to win the European light heavyweight championship.
The stadium became quiet for a profoundly moving memorial prior to the bout as ten bells tolled in memory of the city's native Ricky Hatton, who passed away 46 years old in recently.
It was the premier televised fight night in the city since Hatton's death. As the concluding sound disappeared, the silence became a roar of "there's only one Ricky Hatton".
The action during the bout equaled the feeling. The eventual winner was sent to the canvas by a powerful punch flush on the face in the initial stage, rising shakily to his position on the referee's count.
But the red-colored shorts of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was marked up soon, heavy swelling appearing under his orbital area as the fighters exchanged powerful shots.
Rea, 27, landed power punches but Arthur, thirty-four, fought fire with fire with powerful upward punches as the action hardly diminished over 12 rounds.
The victor barely secured the decision with judges scorecards of draw, 115-113 and 115-112, but the two athletes were determined and courageous, all the qualities their inspiration the legend stood for.
