British light heavyweight Buatsi earned a win yet disappointing beginning to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a majority decision in a competitive ten-round bout at the Manchester venue.
But Parker landed significant shots, delivering effective clean blows in a match where the styles failed to mesh, with frequent clinching spoiling the contest.
The officials judged it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my best performance but what defines champions? They win," the victor said in the ring as he was met by negative reactions from the audience.
The Londoner got back to winning after his initial pro loss - an memorable fight with his previous opponent in February - to claim a 20th pro success.
Several boxing experts felt Parker, thirty-one, deserved the victory but the Derbyshire fighter experienced the second loss of his 28-fight career.
Buatsi Secures Victory in Slow Match
His career development has slowed in recent years through management issues and long spells of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to fulfil that early promise - perhaps in what seems like a last chance - under new promotional backing.
He began cautiously, with the two boxers ready to be aggressive in a calculated opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after damaging his hand mid-fight, was participating in his second fight at light heavyweight.
By the middle rounds, the underdog had kept it close, making life awkward for the favorite, who operated with a solid defense to narrowly win the cleaner stanzas.
The audience's tolerance diminished because of frequent holding, with the challenger going down several times during close exchanges.
Parker ended stronger, but several stanzas were challenging to assess. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, giving Buatsi just a single round.
The victor advances, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Hatton Tribute Inspires Manchester Derby Contest
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a outstanding Manchester vs Manchester fight to win the European light-heavyweight title.
The arena grew hushed for a extremely emotional tribute prior to the bout as a bell ceremony rang in memory of Manchester's own Ricky Hatton, who passed away at age 46 in recently.
It was the premier televised match card in the location since Hatton's death. As the concluding sound disappeared, the stillness transitioned to a shout of "there's just one Hatton".
The fighting during the bout matched the sentiment. Arthur was knocked down by a powerful punch flush on the face in the second round, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red attire of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was marked up early, heavy swelling forming under his face as the duo exchanged punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, 34, responded in kind with powerful upward punches as the action seldom decreased over twelve rounds.
The victor edged the decision with official scores of even score, 115-113 and wider gap, but the two athletes were relentless and courageous, all the qualities their inspiration the legend represented.
