British light-heavyweight Buatsi secured a victory yet disappointing beginning to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a close 10-round bout at the Co-op Live Arena.
But Parker found success, landing some telling clean blows in a match where the styles never quite clicked, with frequent clinching disrupting the fight.
Two judges judged it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest display but what defines champions? They win," the victor said in the ringside as he was faced negative reactions from the audience.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers believed Parker, thirty-one, merited the win but the contender suffered the second defeat of his 28-match career.
The Champion Secures Triumph in Dull Match
Buatsi's progress has plateaued in recent years through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of British boxing, he now is positioned trying to realize his potential - maybe in what seems like a last chance - under fresh management.
He opened tentatively, with both competitors willing to overcommit in a calculated opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand during the bout, was making just his second bout at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, causing problems for the favorite, who operated with a solid defense to barely take the more effective periods.
The spectators' acceptance diminished amid regular grabbing, with the challenger falling to the floor multiple occasions during clinches.
The underdog finished the stronger, but many rounds were challenging to assess. Although, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, giving Buatsi just only one stanza.
Buatsi progresses, but with the likes of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Hatton Homage Motivates Local Rivalry Battle
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a excellent Manchester vs Manchester battle to win the continental title.
The venue became quiet for a profoundly moving homage prior to the bout as a bell ceremony sounded in honor of the city's native Ricky Hatton, who died at age 46 in September.
It was the first major televised fight night in the urban center since Hatton's death. As the final chime disappeared, the silence transitioned to a roar of "there's only one Hatton".
The competition inside the ropes equaled the emotion. Arthur was sent to the canvas by a left hook squarely on the chin in the second round, rising shakily to his footing on the eight count.
But the traditional red equipment of Arthur soon advanced again. From the other corner, Rea - in rival City blue - was damaged early, noticeable damage appearing under his orbital area as the duo exchanged hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the pace seldom decreased over 12 rounds.
The victor narrowly won the verdict with scoring tallies of draw, narrow lead and 115-112, but each competitor were determined and full of heart, exactly what their idol Hatton represented.
