UK light heavyweight Buatsi secured a victory yet underwhelming start to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a majority decision in a close 10-round contest at the boxing arena.
But Parker landed significant shots, connecting with powerful single shots in a match where the techniques failed to mesh, with constant grabbing marring the contest.
The officials had it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my finest showing but what do champions do? They succeed," the victor commented in the ring as he was met by boos from the audience.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in February - to secure a 20th pro success.
Some ringside observers believed Parker, thirty-one, deserved the win but the contender suffered the second loss of his 28-fight professional journey.
Buatsi Claims Triumph in Uneventful Match
His career development has plateaued in recent years through contract problems and long spells of lack of fights.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under recent support.
He started carefully, with neither fighter ready to be aggressive in a tentative initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after damaging his hand mid-fight, was participating in his second bout at light heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for the favorite, who utilized a protective stance to edge the cleaner rounds.
The spectators' acceptance diminished amid regular grabbing, with the challenger going down multiple occasions during clinches.
The underdog closed well, but multiple periods were challenging to assess. Although, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, scoring for Buatsi just one round.
The victor advances, but with the presence of power puncher Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Homage Motivates All-Manchester Battle
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a outstanding Manchester vs Manchester battle to capture the European light heavyweight title.
The stadium grew hushed for a profoundly moving memorial ahead of the contest as ten bells sounded in remembrance of local hero Hatton, who passed away at age 46 in recently.
It was the premier televised boxing event in the city since Hatton's passing. As the final chime disappeared, the quiet gave way to a chant of "only one Ricky Hatton".
The fighting during the bout matched the emotion. The eventual winner was knocked down by a significant blow squarely on the chin in the early round, standing uncertainly to his footing on the referee's count.
But the red-colored attire of the fighter soon pressed ahead again. Across the ring, the opponent - in opposing blue - was injured soon, significant bruising appearing under his eye as the fighters traded hard punches.
Rea, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the action hardly diminished over the full distance.
The victor narrowly won the verdict with official scores of even score, narrow lead and clearer margin, but the two athletes were persistent and full of heart, everything their idol Hatton represented.
