British light heavyweight Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a close 10-round contest at the boxing arena.
But the challenger had his moments, landing some telling individual punches in a match where the techniques failed to mesh, with excessive holding spoiling the contest.
The scoring judges judged it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my best showing but what do champions do? They succeed," Buatsi stated in the arena as he was met by disapproval from the spectators.
The British fighter got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to secure a 20th pro success.
Several boxing experts felt Parker, thirty-one, deserved the decision but the contender suffered the second setback of his 28-fight professional journey.
Buatsi Claims Win in Slow Affair
The fighter's advancement has slowed in recent years through promotional disputes and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now stands trying to fulfil that early promise - maybe in what feels like a last chance - under new promotional backing.
He began cautiously, with both competitors ready to overcommit in a calculated early stages.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after breaking his hand in the contest, was participating in his second bout at light heavyweight.
By the halfway stage, Parker had kept it close, causing problems for the favorite, who utilized a protective stance to edge the better periods.
The spectators' acceptance diminished during regular grabbing, with the challenger falling to the floor multiple occasions during clinches.
Parker ended stronger, but many rounds were difficult to score. Although, former world champion the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the caliber of hard hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Memorial Inspires Manchester Derby Battle
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a outstanding all-Manchester battle to win the European light-heavyweight belt.
The stadium became quiet for a extremely emotional homage ahead of the contest as 10 bells sounded in remembrance of local hero Ricky Hatton, who died 46 years old in September.
It was the premier televised match card in the location since Hatton's passing. As the final chime ended, the silence gave way to a shout of "only one Hatton".
The competition during the bout reflected the sentiment. The eventual winner was knocked down by a left hook squarely on the jaw in the initial stage, getting up unsteadily to his footing 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 marked up quickly, noticeable damage forming under his eye as the duo traded punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with powerful upward punches as the action rarely dropped over 12 rounds.
Arthur edged the result with judges scorecards of draw, close margin and 115-112, but both fighters were relentless and courageous, all the qualities their hero the legend stood for.
