British light-heavyweight Buatsi made a winning yet disappointing beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a tight ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, landing some telling individual punches in a fight where the styles never quite clicked, with constant grabbing disrupting the action.
The officials scored it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest display but how do champions respond? They prevail," Buatsi stated in the ringside as he was faced disapproval from the audience.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to claim a 20th career victory.
Various analysts felt the challenger, thirty-one, should have been awarded the victory but the contender experienced the second loss of his 28-match career.
Buatsi Earns Win in Uneventful Affair
Buatsi's progress has stalled in recent times through management issues and extended periods of lack of fights.
Earlier forecast as the future of the British fight scene, he now stands trying to realize his potential - perhaps in what seems like a final roll of the dice - under recent support.
He began cautiously, with the two boxers prepared to be aggressive in a calculated opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second appearance at light heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who utilized a solid defense to edge the better periods.
The audience's tolerance wore thin because of constant clinching, with Parker slipping to the canvas multiple occasions during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. However, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Tribute Inspires All-Manchester Fight
In the secondary fight, Arthur overcame Rea by divided decision in a terrific Manchester vs Manchester fight to capture the European light heavyweight title.
The arena became quiet for a profoundly moving tribute prior to the bout as ten bells tolled in remembrance of Manchester's own Hatton, who died 46 years old in recently.
It was the premier televised match card in the city since the passing of Hatton. As the last bell faded, the stillness gave way to a shout of "there's just one Hatton".
The action inside the ropes matched the emotion. The eventual winner was floored by a left hook squarely on the chin in the second round, rising shakily to his footing on the referee's count.
But the traditional red equipment of the fighter soon came forward again. On the opposite side, the opponent - in opposing blue - was injured early, noticeable damage appearing under his orbital area as the duo landed punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the pace rarely dropped over twelve rounds.
The victor edged the result with judges scorecards of 114-114, narrow lead and 115-112, but the two athletes were relentless and full of heart, exactly what their inspiration the legend represented.
