British light heavyweight Buatsi made a winning yet disappointing debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a majority decision in a competitive 10-round fight at the Co-op Live Arena.
But the challenger found success, landing some telling individual punches in a contest where the techniques failed to mesh, with excessive holding spoiling the action.
The officials scored it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my finest performance but what do champions do? They prevail," the victor said in the ringside as he was faced disapproval from the audience.
The British fighter got back to winning after his maiden career setback - an thrilling contest with his previous opponent in February - to secure a 20th career victory.
Various analysts believed the challenger, thirty-one, merited the win but the British boxer suffered the second loss of his 28-bout career.
Buatsi Earns Victory in Slow Encounter
The fighter's advancement has plateaued in the past few years through contract problems and extended periods of inactivity.
Once tipped as the future of the British fight scene, he now stands trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under recent support.
He began cautiously, with the two boxers prepared to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second fight at the higher weight class.
By the halfway stage, the underdog had kept it close, creating difficulties for the favorite, who operated with a protective stance to narrowly win the cleaner periods.
The spectators' acceptance diminished during constant clinching, with Parker slipping to the canvas multiple occasions during close exchanges.
Parker ended stronger, but multiple periods were hard to judge. However, ex-champion the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Homage Fuels All-Manchester Contest
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a outstanding local rivalry contest to win the European light heavyweight belt.
The stadium fell silent for a extremely emotional tribute ahead of the contest as a bell ceremony tolled in honor of the city's native Hatton, who passed away aged 46 in recently.
It was the first major televised match card in the city since the passing of Hatton. As the last bell disappeared, the stillness transitioned to a chant of "there's just one Hatton".
The action during the bout equaled the sentiment. Arthur was sent to the canvas by a left hook squarely on the jaw in the initial stage, standing uncertainly to his footing on the eight count.
But the Manchester United-red attire of Arthur soon came forward again. Across the ring, the opponent - in opposing blue - was damaged soon, significant bruising forming under his orbital area as the duo exchanged hard punches.
The challenger, 27, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the pace rarely dropped over 12 rounds.
The victor narrowly won the decision with official scores of even score, close margin and clearer margin, but the two athletes were determined and courageous, everything their hero the legend represented.
