British light-heavyweight Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a split decision in a tight 10-round contest at the Manchester venue.
But the challenger found success, delivering effective single shots in a match where the styles failed to mesh, with constant grabbing spoiling the action.
Two judges judged it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my finest performance but what defines champions? They prevail," Buatsi commented in the arena as he was greeted with disapproval from the crowd.
The Londoner bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to secure a 20th career victory.
Some ringside observers thought Parker, 31, should have been awarded the win but the contender endured the second loss of his 28-match career.
The Champion Earns Victory in Dull Affair
The fighter's advancement has plateaued in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to achieve expectations - possibly in what feels like a last chance - under recent support.
He began cautiously, with neither fighter willing 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 competing in only his second appearance at light-heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for Buatsi, who operated with a solid defense to narrowly win the better rounds.
The crowd's patience decreased during constant clinching, with Parker going down multiple occasions during clinches.
Parker closed well, but many rounds were challenging to assess. Although, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
The victor progresses, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Tribute Inspires Manchester Derby Battle
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester fight to capture the European light heavyweight title.
The venue became quiet for a deeply poignant memorial ahead of the contest as ten bells sounded in memory of Manchester's own Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast fight night in the urban center since Hatton's death. As the last bell ended, the silence transitioned to a chant of "there's just one Ricky Hatton".
The fighting within the ring equaled the sentiment. Arthur was knocked down by a powerful punch directly to the chin in the early round, rising shakily to his position on the referee's count.
But the red-colored shorts of the fighter soon came forward again. From the other corner, the opponent - in competitive blue - was marked up quickly, significant bruising forming under his orbital area as the fighters traded punishing blows.
Rea, 27, delivered effective strikes but the champion, 34, fought fire with fire with effective uppercuts as the pace hardly diminished over the full distance.
The victor narrowly won the verdict with official scores of even score, close margin and clearer margin, but the two athletes were relentless and brave, all the qualities their hero Hatton represented.
