British light-heavyweight Buatsi secured a victory yet disappointing start to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a close ten-round bout at the boxing arena.
But Parker had his moments, landing some telling single shots in a match where the approaches never quite clicked, with frequent clinching disrupting the contest.
The officials scored it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest display but how do champions respond? They prevail," the victor stated in the ringside as he was greeted with disapproval from the crowd.
The British fighter got back to winning after his first professional defeat - an thrilling contest with Callum Smith in last winter - to claim a 20th pro success.
Various analysts felt the challenger, thirty-one, deserved the victory but the British boxer experienced the second setback of his 28-match professional journey.
Buatsi Earns Win in Uneventful Affair
The fighter's advancement has slowed in recent times through promotional disputes and long spells of inactivity.
Once tipped as the future of British boxing, he now is positioned trying to achieve expectations - possibly in what feels like a last chance - under new promotional backing.
He started carefully, with neither fighter willing 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 suffering a hand injury in the contest, was participating in his second bout at the higher weight class.
By the fight's midpoint, Parker had kept it close, making life awkward for Buatsi, who operated with a protective stance to narrowly win the cleaner rounds.
The crowd's patience diminished amid constant clinching, with the challenger going down various instances during grappling.
The underdog ended stronger, but many rounds were hard to judge. Nevertheless, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor advances, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Memorial Fuels All-Manchester Battle
In the co-main event, Lyndon Arthur beat Bradley Rea by close verdict in a terrific all-Manchester contest to secure the continental title.
The venue grew hushed for a deeply poignant tribute ahead of the contest as 10 bells sounded in honor of local hero Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the concluding sound disappeared, the quiet gave way to a chant of "only one Ricky Hatton".
The fighting during the bout equaled the emotion. The eventual winner was floored by a left hook squarely on the jaw in the early round, getting up unsteadily to his position on the count of eight.
But the red-colored shorts of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was marked up early, significant bruising forming under his eye as the pair landed powerful shots.
The challenger, 27, landed power punches but the champion, 34, responded in kind with powerful upward punches as the pace rarely dropped over the full distance.
Arthur narrowly won the result with official scores of even score, close margin and clearer margin, but both fighters were persistent and full of heart, exactly what their hero Hatton embodied.
