UK light-heavyweight Buatsi secured a victory yet unimpressive debut to life under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a tight ten-round contest at the Manchester venue.
But Parker found success, connecting with powerful clean blows in a fight where the styles didn't complement each other, with constant grabbing marring the contest.
The scoring judges judged it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my finest performance but what defines champions? They succeed," Buatsi commented in the arena as he was faced negative reactions from the spectators.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers felt Parker, 31, deserved the decision but the Derbyshire fighter suffered the second loss of his 28-bout career.
The Champion Earns Victory in Uneventful Affair
His career development has stalled in recent years through contract problems and extended periods of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what appears to be a ultimate opportunity - under fresh management.
He opened tentatively, with neither fighter willing to be aggressive in a calculated opening few rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to edge the more effective rounds.
The crowd's patience decreased during constant clinching, with the challenger going down various instances during clinches.
Parker closed well, but several stanzas were difficult to score. Although, previous titleholder Carl Frampton - commentating on Dazn - had it particularly wide for Parker, scoring for Buatsi just one round.
The victor moves on, but with the likes of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Tribute Inspires Local Rivalry Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding all-Manchester battle to secure the continental championship.
The arena fell silent for a extremely emotional tribute prior to the bout as 10 bells tolled in honor of Manchester's own Ricky Hatton, who succumbed aged 46 in September.
It was the premier televised match card in the urban center since the passing of Hatton. As the final chime ended, the silence became a roar of "only one Ricky Hatton".
The competition during the bout matched the feeling. The eventual winner was sent to the canvas by a significant blow squarely on the jaw in the early round, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red shorts of Arthur soon came forward again. From the other corner, Rea - in competitive blue - was injured quickly, noticeable damage appearing under his orbital area as the fighters landed powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the action seldom decreased over the full distance.
Arthur edged the verdict with scoring tallies of draw, close margin and 115-112, but the two athletes were persistent and full of heart, exactly what their inspiration the legend represented.
