UK light heavyweight Buatsi secured a victory yet unimpressive beginning to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a tight 10-round bout at the Manchester venue.
But the challenger had his moments, landing some telling individual punches in a contest where the styles failed to mesh, with constant grabbing disrupting the fight.
The scoring judges had it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my greatest display but what do champions do? They succeed," Buatsi said in the ringside as he was met by negative reactions from the spectators.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a twentieth professional win.
Some ringside observers felt Parker, 31, deserved the win but the Derbyshire fighter suffered the second defeat of his 28-fight career.
The Champion Claims Triumph in Uneventful Encounter
His career development has plateaued in the past few years through contract problems and long spells of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now stands trying to achieve expectations - possibly in what seems like a last chance - under recent support.
He opened tentatively, with neither fighter willing to take risks in a cagey early stages.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second appearance at light-heavyweight.
By the middle rounds, the underdog had made it competitive, causing problems for the favorite, who worked behind a solid defense to barely take the better stanzas.
The crowd's patience wore thin amid regular grabbing, with the challenger slipping to the canvas several times during close exchanges.
Parker ended stronger, but several stanzas were challenging to assess. Although, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the presence of heavy hitter Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Hatton Memorial Inspires Local Rivalry Fight
In the co-main event, Arthur defeated Rea by divided decision in a terrific local rivalry fight to win the continental title.
The venue became quiet for a deeply poignant memorial ahead of the contest as ten bells sounded in remembrance of Manchester's own Hatton, who died 46 years old in September.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the last bell ended, the quiet transitioned to a shout of "only one Ricky Hatton".
The fighting during the bout equaled the feeling. The eventual winner was floored by a significant blow flush on the jaw in the early round, standing uncertainly to his position on the referee's count.
But the red-colored shorts of the fighter soon came forward again. On the opposite side, the opponent - in rival City blue - was damaged quickly, noticeable damage appearing under his orbital area as the pair landed punishing blows.
Rea, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over 12 rounds.
Arthur edged the result with judges scorecards of draw, narrow lead and clearer margin, but each competitor were persistent and courageous, everything their hero Hatton embodied.
