UK light heavyweight Buatsi earned a win yet unimpressive beginning to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a tight 10-round fight at the boxing arena.
But Parker found success, delivering effective clean blows in a contest where the approaches failed to mesh, with excessive holding marring the action.
Two judges scored it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my best showing but how do champions respond? They succeed," the victor commented in the arena as he was faced boos from the audience.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to secure a 20th pro success.
Some ringside observers thought the challenger, 31, should have been awarded the victory but the British boxer endured the second setback of his 28-bout career.
Buatsi Claims Win in Uneventful Encounter
His career development has plateaued in recent years through promotional disputes and considerable time of inactivity.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to realize his potential - perhaps in what seems like a ultimate opportunity - under fresh management.
He started carefully, with the two boxers prepared to take risks in a cagey early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was participating in his second fight at light heavyweight.
By the fight's midpoint, the underdog had kept it close, making life awkward for Buatsi, who operated with a solid defense to barely take the more effective rounds.
The spectators' acceptance decreased because of frequent holding, with Parker slipping to the canvas various instances during grappling.
Parker closed well, but several stanzas were hard to judge. Although, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - on this evidence - still looks steep.
Hatton Memorial Fuels All-Manchester Contest
In the supporting bout, Arthur defeated Rea by divided decision in a terrific Manchester vs Manchester battle to capture the European light heavyweight championship.
The arena fell silent for a profoundly moving homage ahead of the contest as a bell ceremony sounded in memory of Manchester's own Ricky Hatton, who died aged 46 in recently.
It was the first major televised match card in the urban center since Hatton's death. As the final chime faded, the quiet transitioned to a roar of "there's only one Ricky Hatton".
The fighting during the bout matched the sentiment. Arthur was floored by a left hook flush on the chin in the early round, standing uncertainly to his position on the referee's count.
But the traditional red equipment of Arthur soon pressed ahead again. From the other corner, the opponent - in competitive blue - was marked up early, significant bruising forming under his orbital area as the fighters traded powerful shots.
Rea, 27, delivered effective strikes but the champion, 34, fought fire with fire with stinging uppercuts as the pace rarely dropped over twelve rounds.
The victor barely secured the decision with scoring tallies of draw, 115-113 and clearer margin, but both fighters were persistent and full of heart, exactly what their inspiration the legend stood for.
