British light heavyweight Buatsi secured a victory yet disappointing beginning to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a split decision in a competitive 10-round fight at the Manchester venue.
But the challenger had his moments, delivering effective single shots in a match where the styles failed to mesh, with constant grabbing spoiling the contest.
Two judges judged it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my finest showing but what defines champions? They win," the victor commented in the ring as he was faced disapproval from the crowd.
The British fighter got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Several boxing experts believed Parker, 31, deserved the victory but the Derbyshire fighter endured the second setback of his 28-match career.
The Champion Claims Victory in Uneventful Match
Buatsi's progress has stalled in recent times through management issues and considerable time of lack of fights.
Once tipped as the rising talent of British boxing, he now stands trying to realize his potential - perhaps in what seems like a ultimate opportunity - under recent support.
He started carefully, with the two boxers willing to overcommit in a cagey opening few rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was making just his second bout at light-heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for Buatsi, who worked behind a solid defense to barely take the better rounds.
The crowd's patience wore thin during regular grabbing, with the challenger going down several times during grappling.
Parker closed well, but several stanzas were hard to judge. Although, former world champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
The victor progresses, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Tribute Fuels Manchester Derby Contest
In the supporting bout, Arthur beat Bradley Rea by split-decision in a terrific local rivalry fight to secure the continental belt.
The arena grew hushed for a profoundly moving memorial ahead of the contest as ten bells rang in memory of the city's native Hatton, who died aged 46 in September.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the concluding sound disappeared, the silence became a chant of "there's only one Hatton".
The competition within the ring matched the feeling. Arthur was knocked down by a significant blow squarely on the jaw in the initial stage, standing uncertainly to his position on the referee's count.
But the traditional red shorts of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was injured soon, noticeable damage appearing under his face as the duo exchanged powerful shots.
The challenger, 27, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the action seldom decreased over twelve rounds.
Arthur edged the verdict with judges scorecards of draw, close margin and 115-112, but both fighters were persistent and courageous, exactly what their idol Hatton embodied.
