UK light heavyweight Buatsi made a winning yet disappointing start to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a close ten-round bout at the boxing arena.
But Parker found success, landing some telling single shots in a fight where the approaches didn't complement each other, with frequent clinching marring the action.
The scoring judges scored it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my finest showing but how do champions respond? They succeed," the victor said in the ringside as he was faced disapproval from the spectators.
The British fighter got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts believed the challenger, thirty-one, should have been awarded the victory but the British boxer suffered the second setback of his 28-fight career.
Buatsi Earns Win in Slow Match
The fighter's advancement has stalled in the past few years through contract problems and long spells of lack of fights.
Once tipped as the next star of British boxing, he now is positioned trying to realize his potential - possibly in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers prepared to be aggressive in a cagey opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand in the contest, was making just his second appearance at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who operated with a solid defense to barely take the more effective periods.
The spectators' acceptance diminished because of regular grabbing, with Parker slipping to the canvas various instances during grappling.
The underdog closed well, but several stanzas were challenging to assess. However, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, giving Buatsi just a single round.
The victor moves on, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - based on this performance - still appears challenging.
Hatton Tribute Fuels Local Rivalry Battle
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a excellent local rivalry contest to capture the continental title.
The venue became quiet for a extremely emotional memorial prior to the bout as a bell ceremony sounded in remembrance of local hero Ricky Hatton, who succumbed aged 46 in last month.
It was the first major televised match card in the location since Hatton's passing. As the final chime faded, the quiet transitioned to a chant of "there's only one Ricky Hatton".
The fighting within the ring reflected the sentiment. Arthur was knocked down by a left hook flush on the face in the initial stage, rising shakily to his position on the count of eight.
But the red-colored equipment of the fighter soon pressed ahead again. From the other corner, the opponent - in rival City blue - was injured quickly, significant bruising developing under his face as the pair exchanged punishing blows.
The challenger, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the intensity seldom decreased over twelve rounds.
Arthur edged the decision with official scores of even score, narrow lead and 115-112, but both fighters were relentless and courageous, all the qualities their idol the legend represented.
