UK light-heavyweight Buatsi secured a victory yet disappointing debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a close ten-round fight at the Manchester venue.
But Parker landed significant shots, landing some telling individual punches in a contest where the techniques never quite clicked, with frequent clinching spoiling the fight.
Two judges had it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my finest showing but what defines champions? They win," the victor stated in the arena as he was greeted with disapproval from the audience.
The Londoner got back to winning after his maiden career setback - an thrilling contest with his previous opponent in February - to secure a 20th pro success.
Various analysts believed Parker, thirty-one, deserved the victory but the British boxer experienced the second loss of his 28-bout professional journey.
The Champion Earns Win in Slow Match
His career development has stalled in recent times through promotional disputes and long spells of lack of fights.
Earlier forecast as the future of British boxing, he now is positioned trying to achieve expectations - perhaps in what seems like a last chance - under fresh management.
He opened tentatively, with both competitors willing to take risks in a calculated early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in two years ago after damaging his hand mid-fight, was competing in only his second fight at light heavyweight.
By the middle rounds, the underdog had kept it close, making life awkward for the favorite, who worked behind a solid defense to narrowly win the more effective stanzas.
The audience's tolerance decreased during constant clinching, with the challenger slipping to the canvas several times during close exchanges.
Parker finished the stronger, but several stanzas were challenging to assess. Although, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
The victor advances, but with the likes of hard hitter Benavidez, and Russian star Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Tribute Fuels Manchester Derby Fight
In the co-main event, Lyndon Arthur overcame Bradley Rea by close verdict in a excellent all-Manchester battle to capture the European light-heavyweight title.
The stadium became quiet for a extremely emotional homage before the fight as ten bells rang in memory of local hero Hatton, who died 46 years old in recently.
It was the first major televised match card in the urban center since the passing of Hatton. As the final chime ended, the stillness became a shout of "only one Hatton".
The competition inside the ropes reflected the feeling. The eventual winner was floored by a left hook flush on the chin in the initial stage, rising shakily to his feet on the eight count.
But the red-colored shorts of the fighter soon pressed ahead again. Across the ring, Rea - in competitive blue - was damaged quickly, significant bruising developing under his eye as the pair traded hard punches.
Rea, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with powerful upward punches as the action seldom decreased over twelve rounds.
Arthur barely secured the result with scoring tallies of 114-114, narrow lead and wider gap, but each competitor were relentless and courageous, everything their inspiration Hatton represented.
