British light heavyweight Buatsi made a winning yet unimpressive start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a close 10-round bout at the Manchester venue.
But the challenger had his moments, connecting with powerful individual punches in a contest where the styles never quite clicked, with constant grabbing marring the action.
Two judges judged it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my finest performance but what defines champions? They succeed," the victor said in the ringside as he was met by boos from the spectators.
The British fighter returned to winning ways after his initial pro loss - an epic battle with his previous opponent in February - to secure a 20th pro success.
Various analysts thought the challenger, 31, deserved the victory but the contender endured the second loss of his 28-bout professional journey.
Buatsi Claims Victory in Dull Affair
The fighter's advancement has plateaued in recent years through management issues and long spells of inactivity.
Once tipped as the next star of UK boxing, he now is positioned trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under fresh management.
He started carefully, with the two boxers ready to take risks in a tentative opening few rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was making just his second fight at the higher weight class.
By the halfway stage, the underdog had made it competitive, causing problems for the favorite, who worked behind a protective stance to edge the better rounds.
The audience's tolerance diminished during constant clinching, with Parker slipping to the canvas various instances during grappling.
The underdog finished the stronger, but many rounds were difficult to score. However, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor advances, but with the presence of power puncher Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Hatton Memorial Inspires All-Manchester Battle
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a terrific local rivalry battle to win the European light-heavyweight belt.
The venue grew hushed for a deeply poignant homage ahead of the contest as a bell ceremony rang in remembrance of Manchester's own Ricky Hatton, who died at age 46 in September.
It was the first major televised boxing event in the urban center since Hatton's passing. As the concluding sound disappeared, the stillness gave way to a roar of "only one Ricky Hatton".
The action during the bout matched the emotion. Arthur was floored by a left hook flush on the chin in the early round, rising shakily to his feet on the count of eight.
But the red-colored shorts of the fighter soon advanced again. From the other corner, Rea - in rival City blue - was damaged soon, heavy swelling forming under his face as the fighters traded hard punches.
The challenger, 27, connected with strong blows but Arthur, 34, responded in kind with effective uppercuts as the pace seldom decreased over the full distance.
The victor narrowly won the decision with official scores of draw, 115-113 and wider gap, but the two athletes were determined and full of heart, exactly what their inspiration Hatton embodied.
