UK light-heavyweight Buatsi secured a victory yet underwhelming start to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a tight ten-round contest at the boxing arena.
But Parker landed significant shots, landing some telling clean blows in a fight where the approaches failed to mesh, with excessive holding disrupting the contest.
Two judges judged it 96-94 for to the winner, with the third a even score.
"It wasn't my greatest showing but what do champions do? They win," Buatsi said in the ringside as he was greeted with boos from the audience.
The British fighter returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to claim a 20th career victory.
Some ringside observers thought Parker, 31, merited the win but the British boxer experienced the second setback of his 28-fight professional journey.
The Champion Claims Win in Uneventful Affair
Buatsi's progress has slowed in recent years through management issues and long spells of inactivity.
Previously predicted as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with both competitors willing to overcommit in a cagey opening few rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second bout at the higher weight class.
By the fight's midpoint, the underdog had kept it close, making life awkward for the favorite, who worked behind a tight guard to edge the more effective periods.
The audience's tolerance wore thin because of frequent holding, with Parker slipping to the canvas several times during grappling.
Parker finished the stronger, but multiple periods were challenging to assess. Although, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
Buatsi progresses, but with the presence of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Hatton Tribute Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent Manchester vs Manchester contest to capture the European light heavyweight championship.
The stadium became quiet for a deeply poignant homage ahead of the contest as a bell ceremony tolled in honor of Manchester's own Hatton, who died 46 years old in last month.
It was the first major televised match card in the urban center since the passing of Hatton. As the concluding sound disappeared, the silence became a shout of "only one Hatton".
The action during the bout equaled the sentiment. The eventual winner was sent to the canvas by a powerful punch squarely on the jaw in the early round, rising shakily to his feet on the eight count.
But the traditional red shorts of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was damaged early, significant bruising developing under his face as the duo traded powerful shots.
Rea, 27, connected with strong blows but the champion, 34, responded in kind with stinging uppercuts as the action rarely dropped over twelve rounds.
The victor narrowly won the decision with official scores of draw, narrow lead and clearer margin, but each competitor were persistent and brave, everything their hero Hatton represented.
