UK light heavyweight Buatsi secured a victory yet disappointing debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a majority decision in a tight 10-round contest at the Manchester venue.
But the challenger landed significant shots, connecting with powerful clean blows in a match where the approaches failed to mesh, with excessive holding spoiling the fight.
The scoring judges scored it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my best showing but what defines champions? They win," Buatsi commented in the arena as he was met by negative reactions from the spectators.
The Londoner bounced back after his maiden career setback - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts felt the challenger, 31, should have been awarded the victory but the British boxer endured the second loss of his 28-match career.
Buatsi Claims Triumph in Slow Match
The fighter's advancement has slowed in recent times through promotional disputes and considerable time of inactivity.
Earlier forecast as the rising talent of British boxing, he now stands trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors willing to be aggressive in a calculated early stages.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was making just his second appearance at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for Buatsi, who utilized a tight guard to barely take the more effective periods.
The spectators' acceptance decreased amid constant clinching, with Parker slipping to the canvas various instances during clinches.
Parker closed well, but multiple periods were difficult to score. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just a single round.
Buatsi progresses, but with the presence of power puncher Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - based on this performance - still appears challenging.
Hatton Memorial Motivates Local Rivalry Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent all-Manchester fight to win the continental championship.
The arena fell silent for a deeply poignant tribute prior to the bout as ten bells tolled in remembrance of Manchester's own Hatton, who passed away 46 years old in September.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the concluding sound disappeared, the quiet transitioned to a roar of "there's only one Hatton".
The fighting during the bout reflected the emotion. The eventual winner was floored by a powerful punch flush on the chin in the second round, getting up unsteadily to his position on the count of eight.
But the traditional red attire of the fighter soon came forward again. From the other corner, Rea - in opposing blue - was marked up early, heavy swelling appearing under his orbital area as the fighters traded punishing blows.
The challenger, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace hardly diminished over twelve rounds.
Arthur barely secured the decision with official scores of draw, close margin and 115-112, but the two athletes were relentless and full of heart, exactly what their inspiration Hatton embodied.
