British light-heavyweight Buatsi made a winning yet underwhelming debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a close ten-round fight at the Manchester venue.
But the challenger found success, connecting with powerful clean blows in a match where the approaches never quite clicked, with frequent clinching spoiling the action.
The officials had it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my best performance but what defines champions? They win," Buatsi commented in the arena as he was met by negative reactions from the audience.
The British fighter bounced back after his maiden career setback - an epic battle with his previous opponent in February - to secure a 20th pro success.
Various analysts felt Parker, thirty-one, deserved the decision but the British boxer endured the second setback of his 28-fight career.
The Champion Earns Win in Slow Encounter
Buatsi's progress has plateaued in recent years through contract problems and long spells of lack of fights.
Earlier forecast as the next star of British boxing, he now is positioned trying to achieve expectations - perhaps in what feels like a last chance - under new promotional backing.
He opened tentatively, with both competitors ready to be aggressive in a calculated opening few rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in two years ago after damaging his hand during the bout, was competing in only his second appearance at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, causing problems for the favorite, who utilized a tight guard to edge the more effective rounds.
The audience's tolerance wore thin amid constant clinching, with the challenger going down multiple occasions during clinches.
The underdog closed well, but several stanzas were difficult to score. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just one round.
Buatsi progresses, but with the presence of heavy hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Memorial Motivates Manchester Derby Contest
In the co-main event, Lyndon Arthur beat Bradley Rea by split-decision in a excellent all-Manchester battle to capture the continental belt.
The venue became quiet for a profoundly moving tribute ahead of the contest as 10 bells tolled in remembrance of Manchester's own Ricky Hatton, who succumbed 46 years old in recently.
It was the first major televised match card in the location since the passing of Hatton. As the concluding sound ended, the quiet became a roar of "there's just one Hatton".
The action during the bout matched the sentiment. Arthur was sent to the canvas by a left hook flush on the chin in the early round, standing uncertainly to his footing on the count of eight.
But the traditional red shorts of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was injured early, noticeable damage forming under his eye as the duo landed powerful shots.
Rea, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with stinging uppercuts as the intensity hardly diminished over 12 rounds.
Arthur edged the decision with scoring tallies of even score, narrow lead and wider gap, but the two athletes were determined and brave, everything their inspiration the legend embodied.
