UK light heavyweight Buatsi made a winning yet unimpressive debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a split decision in a competitive ten-round contest at the Manchester venue.
But Parker landed significant shots, delivering effective clean blows in a contest where the styles failed to mesh, with excessive holding spoiling the contest.
The officials scored it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my finest display but what defines champions? They prevail," Buatsi commented in the ringside as he was met by disapproval from the crowd.
The Londoner bounced back after his first professional defeat - an epic battle with Callum Smith in February - to earn a twentieth professional win.
Some ringside observers felt the challenger, 31, deserved the decision but the Derbyshire fighter endured the second setback of his 28-match professional journey.
The Champion Earns Victory in Dull Affair
The fighter's advancement has stalled in recent years through management issues and extended periods of lack of fights.
Once tipped as the next star of British boxing, he now finds himself trying to achieve expectations - possibly in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter willing to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury in the contest, was participating in his second appearance at the higher weight class.
By the halfway stage, Parker had kept it close, creating difficulties for Buatsi, who operated with a protective stance to barely take the more effective stanzas.
The crowd's patience diminished because of regular grabbing, with the challenger falling to the floor multiple occasions during clinches.
The underdog closed well, but many rounds were hard to judge. Although, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just a single round.
The victor advances, but with the presence of power puncher Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Memorial Motivates Local Rivalry Fight
In the co-main event, Arthur defeated Rea by split-decision in a excellent Manchester vs Manchester fight to capture the continental championship.
The stadium became quiet for a deeply poignant homage before the fight as a bell ceremony sounded in honor of Manchester's own Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the final chime ended, the stillness gave way to a chant of "there's just one Hatton".
The fighting inside the ropes matched the sentiment. The eventual winner was floored by a powerful punch flush on the face in the early round, standing uncertainly to his position on the eight count.
But the traditional red shorts of the fighter soon advanced again. Across the ring, the opponent - in rival City blue - was marked up early, significant bruising developing under his eye as the duo exchanged hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with powerful upward punches as the intensity rarely dropped over the full distance.
The victor narrowly won the result with scoring tallies of 114-114, 115-113 and wider gap, but both fighters were determined and full of heart, exactly what their idol Hatton embodied.
