British light-heavyweight Buatsi made a winning yet underwhelming beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a majority decision in a close ten-round contest at the Co-op Live Arena.
But Parker found success, delivering effective single shots in a contest where the approaches failed to mesh, with frequent clinching disrupting the contest.
The officials judged it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my greatest display but what do champions do? They win," the victor stated in the ring as he was greeted with negative reactions from the audience.
The Londoner got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Various analysts believed Parker, thirty-one, deserved the win but the Derbyshire fighter suffered the second loss of his 28-bout career.
The Champion Secures Victory in Uneventful Affair
His career development has plateaued in recent times through contract problems and extended periods of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - maybe in what appears to be a last chance - under new promotional backing.
He started carefully, with neither fighter prepared to overcommit in a calculated early stages.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, Parker had made it competitive, creating difficulties for Buatsi, who worked behind a protective stance to barely take the cleaner periods.
The crowd's patience decreased during frequent holding, with Parker going down multiple occasions during grappling.
Parker finished the stronger, but several stanzas were hard to judge. Although, former world champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
The victor advances, but with the presence of hard hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Homage Inspires Local Rivalry Fight
In the co-main event, Lyndon Arthur beat Bradley Rea by close verdict in a excellent all-Manchester battle to capture the European light heavyweight belt.
The stadium grew hushed for a deeply poignant memorial prior to the bout as ten bells rang in memory of the city's native Ricky Hatton, who passed away aged 46 in September.
It was the initial significant broadcast match card in the urban center since the passing of Hatton. As the concluding sound ended, the stillness transitioned to a shout of "there's just one Hatton".
The fighting within the ring reflected the emotion. Arthur was floored by a significant blow squarely on the chin in the initial stage, rising shakily to his position on the referee's count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was damaged early, heavy swelling appearing under his face as the fighters landed punishing blows.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with effective uppercuts as the pace rarely dropped over twelve rounds.
The victor narrowly won the decision with judges scorecards of even score, narrow lead and 115-112, but each competitor were determined and full of heart, all the qualities their idol the legend stood for.
