British light-heavyweight Buatsi made a winning yet unimpressive debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a close 10-round fight at the boxing arena.
But the challenger found success, connecting with powerful single shots in a match where the techniques never quite clicked, with constant grabbing marring the action.
The officials scored it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my greatest performance but how do champions respond? They win," the victor commented in the arena as he was faced boos from the crowd.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers thought Parker, thirty-one, merited the decision but the contender endured the second defeat of his 28-fight professional journey.
The Champion Claims Win in Slow Affair
Buatsi's progress has stalled in recent times through contract problems and considerable time of inactivity.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what feels like a last chance - under new promotional backing.
He began cautiously, with the two boxers ready to take risks in a cagey early stages.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was participating in his second fight at the higher weight class.
By the middle rounds, Parker had kept it close, creating difficulties for Buatsi, who worked behind a tight guard to barely take the better periods.
The spectators' acceptance wore thin during frequent holding, with Parker falling to the floor multiple occasions during grappling.
The underdog finished the stronger, but many rounds were difficult to score. Nevertheless, former world champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the likes of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Homage Fuels All-Manchester Battle
In the supporting bout, Arthur beat Rea by divided decision in a terrific local rivalry fight to capture the European light heavyweight title.
The venue grew hushed for a profoundly moving homage before the fight as a bell ceremony rang in honor of local hero Ricky Hatton, who died aged 46 in last month.
It was the first major televised fight night in the urban center since Hatton's passing. As the final chime disappeared, the silence transitioned to a roar of "there's just one Ricky Hatton".
The fighting during the bout matched the sentiment. The eventual winner was floored by a left hook directly to the chin in the initial stage, getting up unsteadily to his feet on the eight count.
But the traditional red equipment of Arthur soon pressed ahead again. Across the ring, the opponent - in competitive blue - was injured soon, noticeable damage appearing under his eye as the fighters landed hard punches.
Rea, 27, connected with strong blows but Arthur, 34, responded in kind with powerful upward punches as the action rarely dropped over 12 rounds.
The victor narrowly won the result with judges scorecards of even score, close margin and 115-112, but the two athletes were determined and full of heart, exactly what their idol the legend represented.
