British light heavyweight Buatsi secured a victory yet disappointing debut to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a majority decision in a competitive ten-round fight at the Manchester venue.
But Parker had his moments, landing some telling individual punches in a match where the techniques failed to mesh, with frequent clinching disrupting the contest.
The officials scored it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my finest showing but what defines champions? They prevail," Buatsi stated in the ring as he was greeted with disapproval from the crowd.
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 the challenger, 31, should have been awarded the victory but the contender suffered the second setback of his 28-bout professional journey.
The Champion Earns Victory in Dull Affair
Buatsi's progress has plateaued in the past few years through promotional disputes and long spells of lack of fights.
Once tipped as the future of the British fight scene, he now stands trying to achieve expectations - possibly in what seems like a last chance - under fresh management.
He started carefully, with both competitors willing to be aggressive in a calculated early stages.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second appearance at light heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for the favorite, who operated with a tight guard to edge the more effective periods.
The spectators' acceptance wore thin because of constant clinching, with the challenger falling to the floor various instances during grappling.
The underdog closed well, but many rounds were difficult to score. Nevertheless, previous titleholder Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
The victor moves on, but with the presence of power puncher Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Memorial Inspires Local Rivalry Contest
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a excellent all-Manchester fight to secure the continental belt.
The venue fell silent for a profoundly moving memorial before the fight as ten bells tolled in honor of Manchester's own Hatton, who passed away aged 46 in last month.
It was the first major televised boxing event in the urban center since Hatton's passing. As the concluding sound disappeared, the silence gave way to a chant of "there's only one Hatton".
The action within the ring reflected the feeling. The eventual winner was knocked down by a left hook squarely on the chin in the early round, standing uncertainly to his footing on the count of eight.
But the red-colored equipment of the fighter soon advanced again. From the other corner, Rea - in rival City blue - was marked up soon, noticeable damage developing under his eye as the pair landed punishing blows.
The challenger, 27, delivered effective strikes but Arthur, 34, responded in kind with stinging uppercuts as the intensity hardly diminished over 12 rounds.
The victor narrowly won the verdict with scoring tallies of 114-114, close margin and clearer margin, but the two athletes were determined and brave, exactly what their hero Hatton embodied.
