UK light-heavyweight Buatsi earned a win yet unimpressive debut to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a split decision in a close 10-round fight at the boxing arena.
But the challenger found success, delivering effective individual punches in a contest where the techniques didn't complement each other, with excessive holding spoiling the fight.
Two judges had it 96-94 for to the winner, with the third a even score.
"It wasn't my finest performance but what defines champions? They win," Buatsi said in the ringside as he was faced disapproval from the spectators.
The Londoner got back to winning after his initial pro loss - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Several boxing experts thought Parker, thirty-one, merited the win but the contender suffered the second setback of his 28-bout career.
Buatsi Claims Triumph in Slow Affair
Buatsi's progress has slowed in recent years through contract problems and considerable time of inactivity.
Earlier forecast as the rising talent of British boxing, he now stands trying to realize his potential - perhaps in what seems like a last chance - under recent support.
He started carefully, with neither fighter willing to overcommit in a calculated initial rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to edge the cleaner periods.
The spectators' acceptance wore thin because of constant clinching, with the challenger slipping to the canvas various instances during clinches.
Parker finished the stronger, but many rounds were hard to judge. Although, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, giving Buatsi just a single round.
The victor moves on, but with the presence of hard hitter Benavidez, and Russian star Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Homage Motivates Local Rivalry Contest
In the co-main event, Lyndon Arthur beat Bradley Rea by divided decision in a terrific Manchester vs Manchester battle to secure the European light-heavyweight belt.
The stadium fell silent for a deeply poignant tribute ahead of the contest as ten bells sounded in memory of Manchester's own Hatton, who died at age 46 in September.
It was the premier televised boxing event in the location since Hatton's death. As the concluding sound disappeared, the quiet transitioned to a roar of "there's just one Hatton".
The competition during the bout reflected the emotion. Arthur was floored by a significant blow squarely on the chin in the early round, rising shakily to his position on the eight count.
But the Manchester United-red equipment of Arthur soon advanced again. On the opposite side, the opponent - in opposing blue - was injured early, noticeable damage appearing under his eye as the duo landed punishing blows.
Rea, 27, delivered effective strikes but Arthur, 34, matched the intensity with powerful upward punches as the action hardly diminished over the full distance.
The victor edged the verdict with official scores of 114-114, narrow lead and 115-112, but the two athletes were persistent and full of heart, all the qualities their inspiration the legend stood for.
