UK light heavyweight Buatsi earned a win yet disappointing beginning to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a tight ten-round bout at the boxing arena.
But Parker found success, landing some telling clean blows in a fight where the techniques never quite clicked, with excessive holding spoiling the fight.
Two judges scored it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest performance but how do champions respond? They prevail," Buatsi stated in the ring as he was faced disapproval from the audience.
The British fighter returned to winning ways after his maiden career setback - an epic battle with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers felt Parker, thirty-one, deserved the win but the British boxer suffered the second setback of his 28-match career.
Buatsi Claims Victory in Uneventful Encounter
His career development has plateaued in recent years through contract problems and considerable time of lack of fights.
Earlier forecast as the future of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under fresh management.
He started carefully, with neither fighter ready to be aggressive in a calculated early stages.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was making just his second bout at light heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for the favorite, who operated with a solid defense to edge the better stanzas.
The crowd's patience decreased because of constant clinching, with Parker going down various instances during clinches.
The underdog finished the stronger, but several stanzas were difficult to score. Nevertheless, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just one round.
Buatsi advances, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Homage Inspires All-Manchester Battle
In the co-main event, Lyndon Arthur overcame Bradley Rea by divided decision in a excellent local rivalry fight to capture the continental title.
The stadium became quiet for a deeply poignant homage prior to the bout as a bell ceremony rang in honor of local hero Hatton, who passed away at age 46 in last month.
It was the premier televised boxing event in the location since the passing of Hatton. As the final chime ended, the quiet became a shout of "there's only one Ricky Hatton".
The fighting inside the ropes equaled the sentiment. Arthur was sent to the canvas by a left hook flush on the face in the second round, getting up unsteadily to his footing on the count of eight.
But the traditional red equipment of the fighter soon came forward again. On the opposite side, the opponent - in competitive blue - was marked up soon, heavy swelling appearing under his eye as the fighters traded punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with stinging uppercuts as the action hardly diminished over the full distance.
Arthur edged the verdict with judges scorecards of 114-114, close margin and wider gap, but the two athletes were relentless and brave, all the qualities their hero the legend stood for.
