UK light-heavyweight Buatsi secured a victory yet underwhelming beginning to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a competitive ten-round fight at the Manchester venue.
But the challenger landed significant shots, connecting with powerful clean blows in a contest where the styles never quite clicked, with frequent clinching spoiling the contest.
Two judges scored it 96-94 to the winner, with the third a 95-95.
"It wasn't my best showing but how do champions respond? They win," the victor stated in the ring as he was met by boos from the crowd.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers believed the challenger, 31, should have been awarded the win but the British boxer suffered the second setback of his 28-match career.
Buatsi Earns Victory in Uneventful Match
The fighter's advancement has stalled in recent years through management issues and considerable time of lack of fights.
Once tipped as the next star of the British fight scene, he now stands trying to realize his potential - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter prepared to overcommit in a cagey opening few rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand during the bout, was making just his second fight at light-heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a tight guard to barely take the more effective periods.
The crowd's patience wore thin because of regular grabbing, with the challenger slipping to the canvas several times during close exchanges.
Parker closed well, but many rounds were hard to judge. Although, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi advances, but with the presence of power puncher Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Tribute Motivates Manchester Derby Fight
In the secondary fight, Lyndon Arthur defeated Bradley Rea by close verdict in a excellent all-Manchester fight to secure the European light-heavyweight title.
The arena became quiet for a extremely emotional homage before the fight as 10 bells tolled in memory of local hero Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast match card in the location since Hatton's passing. As the concluding sound ended, the stillness gave way to a roar of "there's just one Hatton".
The action inside the ropes reflected the feeling. Arthur was sent to the canvas by a significant blow directly to the face in the initial stage, rising shakily to his feet on the count of eight.
But the red-colored equipment of Arthur soon came forward again. On the opposite side, Rea - in opposing blue - was damaged early, heavy swelling appearing under his face as the fighters landed punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the intensity seldom decreased over the full distance.
Arthur edged the result with scoring tallies of draw, 115-113 and clearer margin, but each competitor were determined and brave, all the qualities their idol Hatton embodied.
