UK light heavyweight Buatsi earned a win yet disappointing start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a tight 10-round fight at the Co-op Live Arena.
But Parker found success, landing some telling individual punches in a fight where the styles failed to mesh, with frequent clinching spoiling the action.
Two judges had it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my finest performance but how do champions respond? They succeed," Buatsi stated in the ringside as he was met by disapproval from the audience.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts believed Parker, 31, merited the victory but the contender experienced the second defeat of his 28-bout career.
Buatsi Claims Victory in Uneventful Affair
Buatsi's progress has slowed in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to realize his potential - maybe in what feels like a last chance - under recent support.
He started carefully, with the two boxers prepared to overcommit in a calculated early stages.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was making just his second fight at the higher weight class.
By the middle rounds, Parker had stayed in contention, causing problems for Buatsi, who operated with a protective stance to barely take the more effective stanzas.
The audience's tolerance decreased during constant clinching, with Parker falling to the floor various instances during grappling.
Parker ended stronger, but multiple periods were hard to judge. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi advances, but with the presence of heavy hitter Benavidez, and Russian champion Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Tribute Inspires Local Rivalry Fight
In the supporting bout, Lyndon Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester contest to win the European light heavyweight championship.
The arena became quiet for a deeply poignant homage prior to the bout as 10 bells sounded in memory of Manchester's own Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised match card in the location since the passing of Hatton. As the last bell faded, the quiet transitioned to a chant of "there's just one Hatton".
The action within the ring matched the sentiment. The eventual winner was sent to the canvas by a significant blow squarely on the jaw in the initial stage, getting up unsteadily to his feet on the referee's count.
But the red-colored shorts of the fighter soon advanced again. Across the ring, the opponent - in competitive blue - was damaged soon, heavy swelling developing under his face as the duo landed punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with effective uppercuts as the pace rarely dropped over 12 rounds.
The victor barely secured the decision with judges scorecards of draw, narrow lead and 115-112, but each competitor were determined and full of heart, all the qualities their inspiration Hatton embodied.
