British light heavyweight Buatsi made a winning yet underwhelming start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a judges' verdict in a competitive 10-round bout at the Co-op Live Arena.
But Parker had his moments, landing some telling single shots in a fight where the approaches never quite clicked, with frequent clinching marring the action.
The scoring judges judged it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my best showing but how do champions respond? They succeed," Buatsi commented in the ring as he was faced boos from the audience.
The British fighter got back to winning after his maiden career setback - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Some ringside observers felt Parker, thirty-one, deserved the victory but the contender experienced the second setback of his 28-match professional journey.
Buatsi Earns Triumph in Slow Match
The fighter's advancement has stalled in recent times through contract problems and extended periods of inactivity.
Once tipped as the rising talent of UK boxing, he now finds himself trying to achieve expectations - perhaps in what seems like a last chance - under recent support.
He began cautiously, with the two boxers ready to take risks in a tentative early stages.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was participating in his second appearance at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, creating difficulties for the favorite, who worked behind a solid defense to edge the cleaner periods.
The crowd's patience wore thin because of constant clinching, with Parker slipping to the canvas several times during close exchanges.
The underdog ended stronger, but several stanzas were hard to judge. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, giving Buatsi just a single round.
Buatsi progresses, but with the likes of power puncher Benavidez, and Russian star Bivol ruling the division, the path to championship success - based on this performance - still appears challenging.
Hatton Memorial Motivates All-Manchester Fight
In the co-main event, Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester fight to capture the European light-heavyweight belt.
The arena grew hushed for a profoundly moving tribute before the fight as 10 bells rang in memory of Manchester's own Ricky Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast match card in the urban center since the passing of Hatton. As the last bell disappeared, the quiet became a roar of "there's only one Ricky Hatton".
The action inside the ropes reflected the emotion. Arthur was floored by a powerful punch flush on the chin in the second round, rising shakily to his position on the referee's count.
But the red-colored shorts of Arthur soon pressed ahead again. On the opposite side, Rea - in rival City blue - was damaged soon, heavy swelling forming under his eye as the pair exchanged punishing blows.
The challenger, 27, delivered effective strikes but the champion, thirty-four, responded in kind with stinging uppercuts as the intensity hardly diminished over twelve rounds.
The victor edged the result with judges scorecards of 114-114, narrow lead and wider gap, but each competitor were determined and full of heart, everything their idol Hatton stood for.
