UK light-heavyweight Buatsi secured a victory yet unimpressive start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a close 10-round fight at the boxing arena.
But the challenger found success, delivering effective clean blows in a match where the techniques never quite clicked, with constant grabbing spoiling the action.
The officials had it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my greatest display but how do champions respond? They succeed," the victor stated in the ringside as he was met by 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 earn a 20th pro success.
Some ringside observers felt the challenger, 31, should have been awarded the decision but the Derbyshire fighter experienced the second defeat of his 28-fight career.
Buatsi Earns Victory in Slow Encounter
His career development has plateaued in recent years through management issues and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under recent support.
He began cautiously, with both competitors willing to take risks in a tentative initial rounds.
The challenger, whose only previous defeat 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 fight's midpoint, the underdog had made it competitive, causing problems for Buatsi, who operated with a solid defense to barely take the better stanzas.
The audience's tolerance decreased amid constant clinching, with the challenger falling to the floor various instances during grappling.
The underdog closed well, but many rounds were challenging to assess. However, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the likes of power puncher Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Tribute Inspires Manchester Derby Battle
In the co-main event, Lyndon Arthur beat Bradley Rea by split-decision in a excellent all-Manchester battle to capture the continental championship.
The venue grew hushed for a deeply poignant homage before the fight as a bell ceremony rang in remembrance of the city's native Ricky Hatton, who died at age 46 in recently.
It was the first major televised boxing event in the city since Hatton's passing. As the last bell disappeared, the silence transitioned to a roar of "only one Ricky Hatton".
The fighting during the bout reflected the sentiment. The eventual winner was knocked down by a significant blow directly to the face in the initial stage, standing uncertainly to his footing on the count of eight.
But the red-colored attire of the fighter soon came forward again. From the other corner, Rea - in opposing blue - was damaged soon, heavy swelling developing under his face as the pair landed hard punches.
The challenger, twenty-seven, landed power punches but the champion, 34, matched the intensity with effective uppercuts as the action seldom decreased over twelve rounds.
The victor edged the result with scoring tallies of 114-114, 115-113 and 115-112, but the two athletes were determined and full of heart, everything their idol the legend represented.
