British light heavyweight Buatsi made a winning yet underwhelming beginning to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a competitive ten-round fight at the Co-op Live Arena.
But Parker had his moments, landing some telling individual punches in a fight where the styles didn't complement each other, with excessive holding spoiling the fight.
Two judges judged it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my greatest display but how do champions respond? They win," the victor stated in the ringside as he was met by boos from the spectators.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in last winter - to secure a 20th career victory.
Several boxing experts believed Parker, thirty-one, should have been awarded the victory but the contender suffered the second loss of his 28-fight career.
The Champion Secures Win in Uneventful Match
His career development has plateaued in recent years through management issues and considerable time of lack of fights.
Previously predicted as the rising talent of UK boxing, he now stands trying to achieve expectations - maybe in what feels like a ultimate opportunity - under recent support.
He started carefully, with both competitors prepared to overcommit in a tentative opening few rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was making just his second bout at the higher weight class.
By the halfway stage, the underdog had kept it close, causing problems for the favorite, who worked behind a solid defense to barely take the better periods.
The spectators' acceptance wore thin amid frequent holding, with Parker falling to the floor various instances during close exchanges.
Parker finished the stronger, but multiple periods were challenging to assess. However, former world champion Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
Buatsi progresses, but with the presence of hard hitter Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - based on this performance - still looks steep.
Hatton Homage Fuels Local Rivalry Contest
In the supporting bout, Lyndon Arthur beat Bradley Rea by close verdict in a outstanding local rivalry fight to win the continental championship.
The venue became quiet for a extremely emotional homage prior to the bout as 10 bells tolled in remembrance of the city's native Hatton, who died 46 years old in September.
It was the first major televised match card in the city since the passing of Hatton. As the final chime ended, the quiet transitioned to a shout of "only one Ricky Hatton".
The competition within the ring matched the emotion. Arthur was knocked down by a significant blow directly to the face in the initial stage, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red attire of Arthur soon came forward again. From the other corner, the opponent - in opposing blue - was injured soon, heavy swelling developing under his eye as the fighters exchanged hard punches.
Rea, 27, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over 12 rounds.
The victor barely secured the verdict with scoring tallies of even score, 115-113 and wider gap, but the two athletes were persistent and brave, exactly what their hero Hatton represented.
