UK light-heavyweight Buatsi secured a victory yet unimpressive debut to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a competitive ten-round fight at the Manchester venue.
But the challenger had his moments, connecting with powerful single shots in a fight where the styles didn't complement each other, with excessive holding marring the fight.
Two judges scored it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my finest display but how do champions respond? They prevail," the victor stated in the ring as he was greeted with negative reactions from the spectators.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in last winter - to earn a twentieth professional win.
Several boxing experts thought the challenger, 31, should have been awarded the victory but the contender endured the second loss of his 28-match career.
The Champion Secures Triumph in Dull Encounter
His career development has plateaued in recent years through contract problems and considerable time of inactivity.
Earlier forecast as the future of British boxing, he now finds himself trying to realize his potential - maybe in what feels like a last chance - under new promotional backing.
He began cautiously, with both competitors ready to be aggressive in a cagey early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was making just his second bout at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, causing problems for the favorite, who utilized a protective stance to edge the better stanzas.
The crowd's patience diminished amid frequent holding, with Parker slipping to the canvas several times during close exchanges.
The underdog finished the stronger, but multiple periods were challenging to assess. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the likes of power puncher David Benavidez, and Russian star Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Hatton Tribute Motivates Manchester Derby Contest
In the co-main event, Lyndon Arthur beat Rea by split-decision in a outstanding Manchester vs Manchester fight to win the European light heavyweight title.
The arena became quiet for a extremely emotional memorial before the fight as 10 bells sounded in honor of the city's native Hatton, who succumbed at age 46 in last month.
It was the first major televised fight night in the city since Hatton's death. As the last bell disappeared, the stillness became a chant of "there's only one Hatton".
The fighting during the bout equaled the emotion. Arthur was sent to the canvas by a significant blow flush on the face in the second round, getting up unsteadily to his footing on the referee's count.
But the traditional red shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in opposing blue - was marked up quickly, significant bruising forming under his eye as the fighters exchanged punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with powerful upward punches as the pace hardly diminished over the full distance.
The victor edged the decision with judges scorecards of draw, narrow lead and wider gap, but the two athletes were determined and brave, all the qualities their idol the legend embodied.
