British light heavyweight Buatsi made a winning yet disappointing beginning to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a competitive ten-round contest at the boxing arena.
But Parker landed significant shots, connecting with powerful individual punches in a contest where the techniques didn't complement each other, with excessive holding spoiling the contest.
Two judges had it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my greatest display but how do champions respond? They prevail," the victor stated in the ring as he was met by disapproval from the audience.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in February - to earn a twentieth professional win.
Various analysts felt Parker, thirty-one, merited the win but the Derbyshire fighter experienced the second loss of his 28-match career.
The Champion Earns Triumph in Dull Match
Buatsi's progress has plateaued in recent years through contract problems and long spells of lack of fights.
Previously predicted as the next star of UK boxing, he now stands trying to realize his potential - possibly in what seems like a final roll of the dice - under fresh management.
He began cautiously, with the two boxers prepared to take risks in a calculated opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second fight at the higher weight class.
By the halfway stage, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a protective stance to edge the more effective stanzas.
The spectators' acceptance diminished because of regular grabbing, with Parker slipping to the canvas various instances during grappling.
Parker ended stronger, but several stanzas were hard to judge. Although, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
Hatton Tribute Motivates All-Manchester Fight
In the supporting bout, Arthur overcame Rea by divided decision in a outstanding local rivalry battle to capture the European light heavyweight championship.
The arena became quiet for a extremely emotional memorial prior to the bout as a bell ceremony tolled in memory of Manchester's own Ricky Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the last bell ended, the quiet became a shout of "there's just one Hatton".
The fighting inside the ropes equaled the sentiment. The eventual winner was knocked down by a left hook flush on the chin in the initial stage, standing uncertainly to his footing on the eight count.
But the Manchester United-red attire of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was marked up quickly, significant bruising appearing under his face as the pair exchanged powerful shots.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with powerful upward punches as the action seldom decreased over twelve rounds.
The victor edged the result with judges scorecards of even score, narrow lead and 115-112, but each competitor were relentless and brave, exactly what their hero Hatton stood for.
