British light-heavyweight Buatsi made a winning yet unimpressive start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a tight ten-round fight at the boxing arena.
But the challenger found success, landing some telling single shots in a contest where the techniques failed to mesh, with excessive holding spoiling the contest.
Two judges judged it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my finest display but how do champions respond? They succeed," the victor stated in the ring as he was greeted with disapproval from the audience.
The British fighter bounced back after his maiden career setback - an memorable fight with his previous opponent in February - to secure a 20th pro success.
Various analysts believed Parker, thirty-one, merited the victory but the Derbyshire fighter suffered the second setback of his 28-fight career.
The Champion Claims Win in Dull Match
Buatsi's progress has plateaued in recent times through contract problems and considerable time of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to realize his potential - possibly in what seems like a ultimate opportunity - under recent support.
He began cautiously, with both competitors ready to be aggressive in a calculated opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand mid-fight, was making just his second fight at light heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for the favorite, who worked behind a tight guard to edge the better stanzas.
The spectators' acceptance diminished during constant clinching, with the challenger going down several times during grappling.
Parker ended stronger, but many rounds were hard to judge. Although, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor advances, but with the caliber of power puncher Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Hatton Memorial Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a outstanding local rivalry contest to win the European light heavyweight belt.
The stadium fell silent for a extremely emotional homage ahead of the contest as 10 bells rang in memory of Manchester's own Ricky Hatton, who succumbed at age 46 in September.
It was the first major televised fight night in the urban center since Hatton's death. As the concluding sound ended, the quiet became a chant of "there's only one Hatton".
The fighting inside the ropes matched the sentiment. Arthur was floored by a significant blow flush on the jaw in the second round, standing uncertainly to his position on the referee's count.
But the red-colored attire of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was marked up early, heavy swelling forming under his eye as the pair landed hard punches.
Rea, 27, landed power punches but the champion, 34, fought fire with fire with powerful upward punches as the intensity seldom decreased over twelve rounds.
Arthur barely secured the result with scoring tallies of draw, close margin and 115-112, but each competitor were relentless and brave, all the qualities their idol the legend embodied.
