UK light heavyweight Buatsi made a winning yet underwhelming beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a competitive 10-round contest at the boxing arena.
But Parker found success, delivering effective clean blows in a match where the techniques never quite clicked, with excessive holding spoiling the contest.
The officials had it 96-94 to the winner, with the third a even score.
"It wasn't my finest performance but how do champions respond? They prevail," the victor said in the ring as he was greeted with boos from the spectators.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a twentieth professional win.
Several boxing experts believed Parker, 31, merited the decision but the Derbyshire fighter suffered the second setback of his 28-match professional journey.
Buatsi Secures Win in Dull Affair
Buatsi's progress has plateaued in the past few years through management issues and long spells of inactivity.
Previously predicted as the next star of the British fight scene, he now is positioned trying to achieve expectations - possibly in what seems like a final roll of the dice - under recent support.
He started carefully, with the two boxers prepared to overcommit in a calculated opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second appearance at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a solid defense to barely take the better stanzas.
The audience's tolerance wore thin because of regular grabbing, with the challenger falling to the floor multiple occasions during close exchanges.
Parker closed well, but many rounds were difficult to score. Although, former world champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
Buatsi moves on, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Tribute Fuels All-Manchester Contest
In the supporting bout, Lyndon Arthur beat Bradley Rea by split-decision in a outstanding all-Manchester fight to capture the European light heavyweight title.
The stadium grew hushed for a extremely emotional tribute ahead of the contest as 10 bells tolled in honor of Manchester's own Hatton, who died at age 46 in last month.
It was the first major televised match card in the city since the passing of Hatton. As the final chime faded, the stillness transitioned to a shout of "there's just one Hatton".
The fighting during the bout equaled the feeling. The eventual winner was sent to the canvas by a significant blow directly to the jaw in the initial stage, rising shakily to his position on the eight count.
But the traditional red shorts of the fighter soon advanced again. Across the ring, Rea - in rival City blue - was injured quickly, noticeable damage developing under his face as the pair landed punishing blows.
The challenger, 27, delivered effective strikes but Arthur, 34, responded in kind with effective uppercuts as the action hardly diminished over 12 rounds.
The victor barely secured the decision with scoring tallies of 114-114, narrow lead and clearer margin, but each competitor were determined and brave, all the qualities their hero Hatton represented.
