UK light-heavyweight Buatsi secured a victory yet disappointing debut to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a tight ten-round bout at the Manchester venue.
But Parker landed significant shots, connecting with powerful clean blows in a contest where the approaches failed to mesh, with frequent clinching disrupting the contest.
The scoring judges scored it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my finest performance but what do champions do? They win," Buatsi said in the ring as he was faced disapproval from the spectators.
The British fighter returned to winning ways after his first professional defeat - an epic battle with Callum Smith in February - to earn a 20th career victory.
Several boxing experts believed the challenger, thirty-one, should have been awarded the win but the contender suffered the second setback of his 28-fight career.
The Champion Earns Triumph in Slow Affair
The fighter's advancement has slowed in the past few years through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of British boxing, he now stands trying to realize his potential - possibly in what feels like a ultimate opportunity - under recent support.
He started carefully, with neither fighter ready to take risks in a tentative early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second fight at light-heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for Buatsi, who worked behind a protective stance to barely take the more effective stanzas.
The spectators' acceptance wore thin during constant clinching, with the challenger falling to the floor multiple occasions during grappling.
The underdog closed well, but many rounds were difficult to score. However, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Hatton Memorial Fuels Manchester Derby Contest
In the co-main event, Lyndon Arthur beat Bradley Rea by divided decision in a excellent Manchester vs Manchester fight to win the European light heavyweight title.
The arena became quiet for a profoundly moving tribute prior to the bout as a bell ceremony sounded in memory of local hero Hatton, who died aged 46 in recently.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the final chime disappeared, the silence gave way to a shout of "there's only one Hatton".
The action during the bout matched the sentiment. Arthur was knocked down by a significant blow squarely on the chin in the initial stage, rising shakily to his footing on the count of eight.
But the red-colored shorts of Arthur soon advanced again. From the other corner, the opponent - in competitive blue - was injured early, significant bruising forming under his face as the pair traded punishing blows.
The challenger, 27, delivered effective strikes but Arthur, 34, matched the intensity with effective uppercuts as the pace hardly diminished over the full distance.
Arthur edged the result with scoring tallies of even score, close margin and 115-112, but each competitor were determined and full of heart, exactly what their hero Hatton represented.
