UK 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 medal winner's activity level likely gained him a split decision in a tight 10-round fight at the boxing arena.
But Parker found success, delivering effective single shots in a fight where the approaches never quite clicked, with constant grabbing marring the action.
Two judges had it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my greatest performance but how do champions respond? They win," the victor said in the ring as he was greeted with boos from the crowd.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in last winter - to claim a twentieth professional win.
Some ringside observers thought the challenger, thirty-one, should have been awarded the victory but the British boxer experienced the second setback of his 28-bout professional journey.
The Champion Earns Win in Uneventful Encounter
The fighter's advancement has stalled in recent times through management issues and extended periods of inactivity.
Once tipped as the next star of UK boxing, he now stands trying to realize his potential - possibly in what feels like a last chance - under fresh management.
He opened tentatively, with both competitors ready to be aggressive in a calculated early stages.
The challenger, whose single earlier setback came at super middleweight against John Ryder in two years ago after damaging his hand in the contest, was making just his second fight at light-heavyweight.
By the middle rounds, the underdog had kept it close, causing problems for the favorite, who utilized a solid defense to narrowly win the better stanzas.
The crowd's patience wore thin because of constant clinching, with Parker going down several times during clinches.
Parker ended stronger, but multiple periods were difficult to score. Although, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
Buatsi moves on, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Hatton Homage Inspires All-Manchester Battle
In the secondary fight, Lyndon Arthur defeated Bradley Rea by split-decision in a terrific Manchester vs Manchester fight to secure the European light heavyweight title.
The arena fell silent for a profoundly moving memorial before the fight as 10 bells tolled in honor of the city's native Ricky Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the final chime faded, the quiet transitioned to a roar of "there's only one Hatton".
The fighting during the bout matched the sentiment. Arthur was sent to the canvas by a left hook directly to the chin in the second round, getting up unsteadily to his footing on the eight count.
But the traditional red shorts of the fighter soon pressed ahead again. Across the ring, the opponent - in rival City blue - was marked up soon, significant bruising developing under his eye as the duo landed hard punches.
Rea, 27, connected with strong blows but the champion, 34, fought fire with fire with stinging uppercuts as the action seldom decreased over 12 rounds.
The victor barely secured the verdict with scoring tallies of even score, close margin and clearer margin, but both fighters were determined and full of heart, all the qualities their idol the legend stood for.
