British light-heavyweight Buatsi made a winning yet disappointing start to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a split decision in a tight 10-round contest at the boxing arena.
But Parker had his moments, landing some telling clean blows in a match where the approaches never quite clicked, with constant grabbing disrupting the fight.
Two judges had it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my greatest showing but what defines champions? They prevail," Buatsi commented in the ringside as he was faced boos from the spectators.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a twentieth professional win.
Some ringside observers believed Parker, 31, should have been awarded the decision but the Derbyshire fighter endured the second setback of his 28-bout career.
Buatsi Earns Triumph in Dull Match
Buatsi's progress has plateaued in recent years through management issues and considerable time of inactivity.
Earlier forecast as the future of UK boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers ready to overcommit in a tentative early stages.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second fight at light heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for the favorite, who worked behind a protective stance to edge the better periods.
The crowd's patience diminished because of frequent holding, with Parker slipping to the canvas multiple occasions during clinches.
Parker ended stronger, but many rounds were hard to judge. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the caliber of heavy hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Tribute Motivates Manchester Derby Battle
In the secondary fight, Arthur defeated Bradley Rea by split-decision in a terrific all-Manchester contest to capture the continental belt.
The stadium fell silent for a extremely emotional homage before the fight as 10 bells sounded in memory of local hero Ricky Hatton, who passed away 46 years old in last month.
It was the first major televised boxing event in the location since Hatton's passing. As the concluding sound faded, the silence transitioned to a chant of "there's just one Ricky Hatton".
The fighting inside the ropes matched the feeling. Arthur was floored by a powerful punch directly to the face in the initial stage, rising shakily to his position on the eight count.
But the red-colored equipment of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was injured quickly, noticeable damage developing under his face as the fighters exchanged punishing blows.
The challenger, 27, connected with strong blows but Arthur, 34, matched the intensity with stinging uppercuts as the pace seldom decreased over the full distance.
Arthur edged the decision with scoring tallies of 114-114, narrow lead and wider gap, but each competitor were determined and courageous, exactly what their inspiration Hatton stood for.
